diff --git a/app/main/views/invites.py b/app/main/views/invites.py
index 43e994e06..2da96a154 100644
--- a/app/main/views/invites.py
+++ b/app/main/views/invites.py
@@ -38,7 +38,7 @@ def accept_invite(token):
 
     invited_user = invite_api_client.check_token(token)
 
-    if not current_user.is_anonymous and current_user.email_address != invited_user.email_address:
+    if not current_user.is_anonymous and current_user.email_address.lower() != invited_user.email_address.lower():
         message = Markup("""
             You’re signed in as {}.
             This invite is for another email address.
diff --git a/app/main/views/sign_in.py b/app/main/views/sign_in.py
index eb77ad7a7..20033afa0 100644
--- a/app/main/views/sign_in.py
+++ b/app/main/views/sign_in.py
@@ -40,7 +40,7 @@ def sign_in():
 
         if user and session.get('invited_user'):
             invited_user = session.get('invited_user')
-            if user.email_address != invited_user['email_address']:
+            if user.email_address.lower() != invited_user['email_address'].lower():
                 flash("You can't accept an invite for another person.")
                 session.pop('invited_user', None)
                 abort(403)
diff --git a/tests/app/main/views/test_accept_invite.py b/tests/app/main/views/test_accept_invite.py
index ae94139c8..0ff67f30b 100644
--- a/tests/app/main/views/test_accept_invite.py
+++ b/tests/app/main/views/test_accept_invite.py
@@ -315,6 +315,29 @@ def test_signed_in_existing_user_cannot_use_anothers_invite(
     assert mock_accept_invite.call_count == 0
 
 
+def test_accept_invite_does_not_treat_email_addresses_as_case_sensitive(
+    logged_in_client,
+    mocker,
+    api_user_active,
+    sample_invite,
+    service_one,
+    mock_accept_invite,
+    mock_get_user_by_email
+):
+    mocker.patch('app.main.views.invites.check_token')
+
+    # the email address of api_user_active is 'test@user.gov.uk'
+    sample_invite['email_address'] = 'TEST@user.gov.uk'
+    invite = InvitedUser(**sample_invite)
+    mocker.patch('app.invite_api_client.check_token', return_value=invite)
+    mocker.patch('app.user_api_client.get_users_for_service', return_value=[api_user_active])
+
+    response = logged_in_client.get(url_for('main.accept_invite', token='thisisnotarealtoken'))
+
+    assert response.status_code == 302
+    assert response.location == url_for('main.service_dashboard', service_id=service_one['id'], _external=True)
+
+
 def test_new_invited_user_verifies_and_added_to_service(
     client,
     service_one,
diff --git a/tests/app/main/views/test_sign_in.py b/tests/app/main/views/test_sign_in.py
index 6f44c8d4a..563d29751 100644
--- a/tests/app/main/views/test_sign_in.py
+++ b/tests/app/main/views/test_sign_in.py
@@ -170,3 +170,30 @@ def test_should_attempt_redirect_when_user_is_pending(
             'password': 'val1dPassw0rd!'})
     assert response.location == url_for('main.resend_email_verification', _external=True)
     assert response.status_code == 302
+
+
+def test_email_address_is_treated_case_insensitively_when_signing_in_as_invited_user(
+    client,
+    mocker,
+    mock_verify_password,
+    api_user_active,
+    sample_invite,
+    mock_accept_invite,
+    mock_send_verify_code
+):
+    sample_invite['email_address'] = 'TEST@user.gov.uk'
+
+    mocker.patch('app.user_api_client.get_user_by_email_or_none', return_value=api_user_active)
+    mocker.patch('app.main.views.sign_in._get_and_verify_user', return_value=api_user_active)
+
+    with client.session_transaction() as session:
+        session['invited_user'] = sample_invite
+
+    response = client.post(
+        url_for('main.sign_in'), data={
+            'email_address': 'test@user.gov.uk',
+            'password': 'val1dPassw0rd!'})
+
+    assert mock_accept_invite.called
+    assert response.status_code == 302
+    assert mock_send_verify_code.called