diff --git a/app/error_messages.py b/app/error_messages.py
index 188a9947d..de038f26e 100644
--- a/app/error_messages.py
+++ b/app/error_messages.py
@@ -1,6 +1,9 @@
 """Error messages used in validators and exceptions throughout the validator codebase."""
 
 DUMB_QUOTES_FOUND = "Found dumb quotes(s) in schema text"
+INVALID_HTML_FOUND = "Found invalid HTML tag(s) in schema text"
+INVALID_HTML_ENTITIES_FOUND = "Found invalid HTML entity(ies) in schema text"
+SPACE_BEFORE_BR = "Found whitespace before <br> tag"
 INVALID_WHITESPACE_FOUND = "Found invalid white space(s) in schema text"
 DUPLICATE_ID_FOUND = "Duplicate id found"
 FOR_LIST_NEVER_POPULATED = "for_list is not populated by any ListCollector blocks or supplementary data sources"
diff --git a/app/validators/questionnaire_validator.py b/app/validators/questionnaire_validator.py
index 27626c94c..bb1ea799d 100644
--- a/app/validators/questionnaire_validator.py
+++ b/app/validators/questionnaire_validator.py
@@ -5,6 +5,7 @@
     QuestionnaireValidator
 """
 
+import html.entities
 import re
 from collections.abc import Mapping
 
@@ -41,6 +42,12 @@ class QuestionnaireValidator(Validator):
         validate_smart_quotes
         validate_white_spaces
         validate_introduction_block
+        validate_html
+        validate_html_tags
+        validate_html_entities
+        is_valid_html_entity
+        validate_whitespace_before_br_tag
+        validate_p_tag_position
         validate_answer_references
         validate_list_references
         resolve_source_block_id
@@ -70,6 +77,7 @@ def validate(self):
         self.validate_duplicates()
         self.validate_smart_quotes()
         self.validate_white_spaces()
+        self.validate_html()
         self.validate_answer_references()
         self.validate_list_references()
 
@@ -167,6 +175,161 @@ def validate_smart_quotes(self):
                         pointer=translatable_item.pointer,
                     )
 
+    def validate_html(self):
+        """Validates HTML in translatable schema text.
+        Checks tags, entities, <br> whitespace, and <p> positioning.
+        """
+        for translatable_item in get_translatable_items(self.schema_element):  # type: ignore
+            schema_text = translatable_item.value
+            values_to_check = [schema_text]
+
+            if isinstance(schema_text, dict):
+                values_to_check = schema_text.values()
+
+            for text in values_to_check:
+                if not isinstance(text, str) or not text:
+                    continue
+
+                if "<" in text and ">" in text:
+                    self.validate_html_tags(text, translatable_item.pointer)
+                    self.validate_whitespace_before_br_tag(text, translatable_item.pointer)
+                    self.validate_p_tag_position(text, translatable_item.pointer)
+
+                if "&" in text and ";" in text:
+                    self.validate_html_entities(text, translatable_item.pointer)
+
+    def validate_html_tags(self, text, pointer):
+        """Validates HTML tags.
+
+        Args:
+            text (str): The text to be validated for HTML tags.
+            pointer (str): The JSON pointer indicating the location of the text in the questionnaire schema, used for
+            error reporting.
+        """
+        allowed_tags = {"p", "strong", "a", "b", "br", "img"}
+        self_closing_tags = {"br", "img"}
+
+        tag_matches = re.finditer(r"</?([a-zA-Z0-9]+)[^>]*>", text)
+        stack = []
+
+        for match in tag_matches:
+            raw_tag = match.group(0)
+            tag_name = match.group(1).lower()
+
+            is_closing = raw_tag.startswith("</")
+            is_self_closing = raw_tag.endswith("/>") or tag_name in self_closing_tags
+
+            if tag_name not in allowed_tags:
+                self.add_error(
+                    error_messages.INVALID_HTML_FOUND,
+                    pointer=pointer,
+                    text=text,
+                )
+                return
+
+            if is_closing:
+                if tag_name in self_closing_tags or not stack or stack[-1] != tag_name:
+                    self.add_error(
+                        error_messages.INVALID_HTML_FOUND,
+                        pointer=pointer,
+                        text=text,
+                    )
+                    return
+
+                stack.pop()
+
+            elif not is_self_closing:
+                stack.append(tag_name)
+
+        if stack:
+            self.add_error(
+                error_messages.INVALID_HTML_FOUND,
+                pointer=pointer,
+                text=text,
+            )
+
+    def is_valid_html_entity(self, entity):
+        """Checks whether a given HTML entity is valid.
+        Supports both numeric (decimal and hexadecimal) and named entities.
+
+        Args:
+            entity (str): The HTML entity to validate (e.g. "&amp;", "&#169;").
+        """
+        if entity.startswith("&#") and entity.endswith(";"):
+            numeric = entity[2:-1]
+
+            is_hex = numeric.lower().startswith("x")
+
+            try:
+                numeric_value = int(numeric[1:], 16) if is_hex else int(numeric)
+            except ValueError:
+                return False
+
+            return 0 <= numeric_value <= 0x10FFFF
+
+        if entity.startswith("&") and entity.endswith(";"):
+            return entity[1:] in html.entities.html5
+
+        return False
+
+    def validate_html_entities(self, text, pointer):
+        """Validates HTML entities found in the text.
+
+        Extracts all entities and checks whether each one is valid.
+
+        Args:
+            text (str): The text to validate for HTML entities.
+            pointer (str): JSON pointer to the location of the text in the schema.
+        """
+        entity_matches = re.findall(r"&[^;\s]+;", text)
+
+        for entity in entity_matches:
+            if not self.is_valid_html_entity(entity):
+                self.add_error(
+                    error_messages.INVALID_HTML_ENTITIES_FOUND,
+                    pointer=pointer,
+                    text=text,
+                )
+                return
+
+    def validate_whitespace_before_br_tag(self, text, pointer):
+        """Checks for invalid whitespace before <br> tags.
+
+        Args:
+            text (str): The text to validate.
+            pointer (str): JSON pointer to the location of the text in the schema.
+        """
+        if re.search(r"\s+<br\s*/?>", text):
+            self.add_error(
+                error_messages.SPACE_BEFORE_BR,
+                pointer=pointer,
+                text=text,
+            )
+
+    def validate_p_tag_position(self, text, pointer):
+        """Checks if p tag is at the start of a sentence
+        (ignoring whitespace and wrapper characters like [] and ()).
+
+        Args:
+            text (str): The text to validate.
+            pointer (str): JSON pointer to the location of the text in the schema.
+        """
+        match = re.search(r"<p(?=[\s>])[^>]*>", text)
+
+        if not match:
+            return
+
+        text_before_p_tag = text[: match.start()]
+
+        content_before_p_tag = text_before_p_tag.strip(" \t\n\r[]()")
+
+        if content_before_p_tag:
+            self.add_error(
+                error_messages.INVALID_HTML_FOUND,
+                pointer=pointer,
+                text=text,
+            )
+
     def validate_white_spaces(self):
         """Validate that there are no leading, trailing or multiple consecutive white spaces in the translatable text
         of the questionnaire schema.
diff --git a/tests/schemas/invalid/test_invalid_html_tags.json b/tests/schemas/invalid/test_invalid_html_tags.json
new file mode 100644
index 000000000..a6255f06d
--- /dev/null
+++ b/tests/schemas/invalid/test_invalid_html_tags.json
@@ -0,0 +1,133 @@
+{
+  "mime_type": "application/json/ons/eq",
+  "language": "en",
+  "schema_version": "0.0.1",
+  "data_version": "0.0.3",
+  "survey_id": "144",
+  "theme": "default",
+  "title": "Test invalid html",
+  "legal_basis": "Notice is given under section 999 of the Test Act 2000",
+  "metadata": [
+    {
+      "name": "user_id",
+      "type": "string"
+    },
+    {
+      "name": "period_id",
+      "type": "string"
+    },
+    {
+      "name": "ru_name",
+      "type": "string"
+    },
+    {
+      "name": "ru_ref",
+      "type": "string"
+    },
+    {
+      "name": "trad_as",
+      "type": "string",
+      "optional": true
+    }
+  ],
+  "questionnaire_flow": {
+    "type": "Linear",
+    "options": {}
+  },
+  "sections": [
+    {
+      "id": "introduction-section",
+      "title": "Introduction <br>",
+      "groups": [
+        {
+          "id": "introduction-group",
+          "title": "<p>General Business Information",
+          "blocks": [
+            {
+              "id": "introduction",
+              "type": "Introduction",
+              "primary_content": [
+                {
+                  "id": "business-details",
+                  "title": "Introduction with &fake; valid and invalid HTML",
+                  "contents": [
+                    {
+                      "guidance": {
+                        "contents": [
+                          {
+                            "title": "<invalid>Coronavirus (COVID-19) guidance</invalid>",
+                            "description": "<strong>Explain your figures</strong> in the comment section to minimise us &amp; contacting you and to help us tell an industry story"
+                          }
+                        ]
+                      }
+                    }
+                  ]
+                }
+              ]
+            },
+            {
+              "type": "Interstitial",
+              "id": "intersitital-one",
+              "content": {
+                "title": "Page <p>with invalid html</p> in title",
+                "contents": [
+                  {
+                    "description": "[<p>You have successfully completed this section</p>]"
+                  }
+                ]
+              }
+            },
+            {
+              "type": "Interstitial",
+              "id": "interstitial-two",
+              "content": {
+                "title": "Page with link",
+                "contents": [
+                  {
+                    "description": "<a href='link'>Anchor</a>"
+                  }
+                ]
+              }
+            },
+            {
+              "type": "Interstitial",
+              "id": "interstitial-three",
+              "content": {
+                "title": "<strong>Page with mixed invalid tags</strong>",
+                "contents": [
+                  {
+                    "description": "<h1>Title</h1><em>Not valid tag</em>"
+                  }
+                ]
+              }
+            },
+            {
+              "type": "Interstitial",
+              "id": "interstitial-four",
+              "content": {
+                "title": "Valid double <strong>strong</strong> with another <strong>strong</strong>.",
+                "contents": [
+                  {
+                    "description": "<strong>Title</strong><em>Not valid tag</em>"
+                  }
+                ]
+              }
+            },
+            {
+              "type": "Interstitial",
+              "id": "interstitial-five",
+              "content": {
+                "title": "Valid double anchor.",
+                "contents": [
+                  {
+                    "description": "<a href='link'>Title</a> and <a href='link-2'>valid tag</a>"
+                  }
+                ]
+              }
+            }
+          ]
+        }
+      ]
+    }
+  ]
+}
diff --git a/tests/test_questionnaire_validator.py b/tests/test_questionnaire_validator.py
index 689ca3afa..a42fd7034 100644
--- a/tests/test_questionnaire_validator.py
+++ b/tests/test_questionnaire_validator.py
@@ -334,6 +334,52 @@ def test_invalid_whitespaces_in_schema():
     assert validator.errors == expected_error_messages
 
 
+def test_invalid_html_in_schema():
+    filename = "schemas/invalid/test_invalid_html_tags.json"
+    validator = QuestionnaireValidator(_open_and_load_schema_file(filename))
+
+    expected_error_messages = [
+        {
+            "message": error_messages.SPACE_BEFORE_BR,
+            "pointer": "/sections/0/title",
+            "text": "Introduction <br>",
+        },
+        {
+            "message": error_messages.INVALID_HTML_FOUND,
+            "pointer": "/sections/0/groups/0/title",
+            "text": "<p>General Business Information",
+        },
+        {
+            "message": error_messages.INVALID_HTML_FOUND,
+            "pointer": "/sections/0/groups/0/blocks/1/content/title",
+            "text": "Page <p>with invalid html</p> in title",
+        },
+        {
+            "message": error_messages.INVALID_HTML_FOUND,
+            "pointer": "/sections/0/groups/0/blocks/3/content/contents/0/description",
+            "text": "<h1>Title</h1><em>Not valid tag</em>",
+        },
+        {
+            "message": error_messages.INVALID_HTML_FOUND,
+            "pointer": "/sections/0/groups/0/blocks/4/content/contents/0/description",
+            "text": "<strong>Title</strong><em>Not valid tag</em>",
+        },
+        {
+            "message": error_messages.INVALID_HTML_ENTITIES_FOUND,
+            "pointer": "/sections/0/groups/0/blocks/0/primary_content/0/title",
+            "text": "Introduction with &fake; valid and invalid HTML",
+        },
+        {
+            "message": error_messages.INVALID_HTML_FOUND,
+            "pointer": "/sections/0/groups/0/blocks/0/primary_content/0/contents/0/guidance/contents/0/title",
+            "text": "<invalid>Coronavirus (COVID-19) guidance</invalid>",
+        },
+    ]
+    validator.validate_html()
+
+    assert validator.errors == expected_error_messages
+
+
 def test_invalid_answer_type_for_question_summary_concatenation():
     filename = "schemas/invalid/test_invalid_answer_type_for_question_summary.json"