diff --git a/app/main/forms.py b/app/main/forms.py
index 10ba612d0c..044669c3dc 100644
--- a/app/main/forms.py
+++ b/app/main/forms.py
@@ -75,6 +75,7 @@
     sentence_case,
 )
 from app.main.validators import (
+    CanEncode,
     CannotContainURLsOrLinks,
     CharactersNotAllowed,
     CommonlyUsedPassword,
@@ -2532,11 +2533,16 @@ class CallbackForm(StripWhitespaceForm):
                 r"(?:#[\w\-._~%!$&'()*+,;=:@/?]*)?$",
                 message="Must be a valid https URL",
             ),
+            CanEncode(field_type="a web address"),
         ],
     )
     bearer_token = GovukPasswordField(
         "Bearer token",
-        validators=[DataRequired(message="Cannot be empty"), Length(min=10, thing="the bearer token")],
+        validators=[
+            DataRequired(message="Cannot be empty"),
+            Length(min=10, thing="the bearer token"),
+            CanEncode(field_type="a bearer token"),
+        ],
     )
 
     def validate(self, *args, **kwargs):
diff --git a/app/main/validators.py b/app/main/validators.py
index 0124c8a2b9..0cb511d3fb 100644
--- a/app/main/validators.py
+++ b/app/main/validators.py
@@ -24,6 +24,46 @@
 from app.utils.user import is_gov_user
 
 
+class CanEncode:
+    """
+    Validates that the field data can be encoded into a specific character set.
+    """
+
+    def __init__(self, encoding="latin-1", field_type=None, message=None):
+        self.encoding = encoding
+        self.field_type = field_type
+        self.message = message
+
+    def __call__(self, form, field):
+        if field.data:
+            unsupported = set()
+            for char in field.data:
+                try:
+                    char.encode(self.encoding)
+                except UnicodeEncodeError:
+                    unsupported.add(char)
+            unsupported_char_list = list(unsupported)
+            if unsupported_char_list:
+                unsupported_char_list.sort()
+
+            field_type = "this field"
+            if self.field_type is not None:
+                field_type = self.field_type
+
+            if unsupported_char_list != []:
+                message = self.message
+                if message is None:
+                    message = (
+                        "You cannot use {} in {}. You must use percent encoding if you want to include {}.".format(
+                            formatted_list(unsupported_char_list, conjunction="or", before_each="", after_each=""),
+                            field_type,
+                            "these characters" if len(unsupported_char_list) > 1 else "this character",
+                        )
+                    )
+
+                raise ValidationError(message)
+
+
 class CommonlyUsedPassword:
     def __init__(self, message=None):
         if not message:
diff --git a/tests/app/main/test_forms.py b/tests/app/main/test_forms.py
index cc5853aff3..9fb3542bf0 100644
--- a/tests/app/main/test_forms.py
+++ b/tests/app/main/test_forms.py
@@ -1,6 +1,7 @@
 import pytest
 
-from app.main.forms import OrderableFieldsForm, StripWhitespaceStringField
+from app.main.forms import CallbackForm, OrderableFieldsForm, StripWhitespaceStringField
+from app.main.validators import CanEncode
 from tests.conftest import set_config_values
 
 
@@ -41,3 +42,9 @@ class TestForm(OrderableFieldsForm):
         with set_config_values(notify_admin, {"WTF_CSRF_ENABLED": True}):
             form = TestForm()
             assert [field.name for field in form] == ["csrf_token", "field2", "field1"]
+
+
+def test_callbackform_has_can_encode_validators(notify_admin, client_request):
+    cbf = CallbackForm()
+    assert any(isinstance(x, CanEncode) for x in cbf.url.validators)
+    assert any(isinstance(x, CanEncode) for x in cbf.bearer_token.validators)
diff --git a/tests/app/main/test_validators.py b/tests/app/main/test_validators.py
index 1f63e6a11c..ffd3bbc143 100644
--- a/tests/app/main/test_validators.py
+++ b/tests/app/main/test_validators.py
@@ -5,6 +5,7 @@
 from wtforms import ValidationError
 
 from app.main.validators import (
+    CanEncode,
     CharactersNotAllowed,
     MustContainAlphanumericCharacters,
     NoCommasInPlaceHolders,
@@ -217,3 +218,39 @@ def test_string_cannot_contain_string_with_custom_error_message():
 
     assert str(error.value) == "No sequences please"
     assert mock_field.error_summary_messages == ["No sequences in %s please"]
+
+
+@pytest.mark.parametrize(
+    "data, err_msg",
+    [
+        (
+            "📵 ghi",
+            "You cannot use 📵 in this field. You must use percent encoding if you want to include this character.",
+        ),
+        (
+            "∆ abc 📲",
+            "You cannot use ∆ or 📲 in this field. You must use percent encoding if you want to include these characters.",  # noqa
+        ),
+    ],
+)
+def test_can_encode_validation(data, err_msg, client_request):
+    with pytest.raises(ValidationError) as error:
+        CanEncode()(None, _gen_mock_field(data))
+
+    assert str(error.value) == err_msg
+
+
+def test_string_can_encode_with_custom_field_type():
+    mock_field = _gen_mock_field("∆ abc 📲", error_summary_messages=[])
+    with pytest.raises(ValidationError) as error:
+        CanEncode(field_type="a web address")(None, mock_field)
+
+    assert (
+        str(error.value)
+        == "You cannot use ∆ or 📲 in a web address. You must use percent encoding if you want to include these characters."  # noqa
+    )
+
+
+@pytest.mark.parametrize("string", ["", "Résumé", "München"])
+def test_string_can_encode_does_not_raise(string):
+    CanEncode()(None, _gen_mock_field(string))