diff --git a/app/main/forms.py b/app/main/forms.py
index e4559a164..9259f5c6f 100644
--- a/app/main/forms.py
+++ b/app/main/forms.py
@@ -1,3 +1,4 @@
+import re
 from flask_wtf import Form
 
 from wtforms import (
@@ -24,6 +25,45 @@ def email_address():
         Regexp(regex=gov_uk_email, message='Enter a gov.uk email address')])
 
 
+class UKMobileNumber(StringField):
+
+    def pre_validate(self, form):
+
+        self.data = self.data.replace('(', '')
+        self.data = self.data.replace(')', '')
+        self.data = self.data.replace(' ', '')
+        self.data = self.data.replace('-', '')
+
+        if self.data.startswith('+'):
+            self.data = self.data[1:]
+
+        if not sum([
+            self.data.startswith(prefix) for prefix in ['07', '447', '4407', '00447']
+        ]):
+            raise ValidationError('Must be a mobile number')
+
+        for digit in self.data:
+            try:
+                int(digit)
+            except(ValueError):
+                raise ValidationError('Must not contain letters or symbols')
+
+        self.data = self.data.split('7', 1)[1]
+
+        if len(self.data) > 9:
+            raise ValidationError('Too many digits')
+
+        if len(self.data) < 9:
+            raise ValidationError('Too few digits')
+
+    def post_validate(self, form, validation_stopped):
+
+        if len(self.data) != 9:
+            return
+
+        self.data = '+44 7{} {} {}'.format(*re.findall('...', self.data))
+
+
 def mobile_number():
     mobile_number_regex = "^\\+44[\\d]{10}$"
     return StringField('Mobile phone number',
diff --git a/tests/app/main/test_phone_number_form_field.py b/tests/app/main/test_phone_number_form_field.py
new file mode 100644
index 000000000..a6994fce5
--- /dev/null
+++ b/tests/app/main/test_phone_number_form_field.py
@@ -0,0 +1,68 @@
+import pytest
+from wtforms import Form
+from app.main.forms import UKMobileNumber
+
+
+class FormExample(Form):
+    phone_number = UKMobileNumber()
+
+phone_numbers = {
+    'invalid': [
+        # Too long
+        '0712345678910',
+        '0044712345678910',
+        '0044712345678910',
+        '+44 (0)7123 456 789 10',
+        # Too short
+        '0712345678',
+        '004471234567',
+        '00447123456',
+        '+44 (0)7123 456 78',
+        # Not mobile (from https://fakenumber.org/generator/freephone)
+        '08081 570364',
+        '+44 8081 570364',
+        '0117 496 0860',
+        '+44 117 496 0860',
+        '020 7946 0991',
+        '+44 20 7946 0991',
+        # Contains non-numbers
+        '07890x32109',
+        '07123 456789...',
+        '07123 ☟☜⬇⬆☞☝',
+        '07123☟☜⬇⬆☞☝',
+        '07";DROP TABLE;"',
+        '+44 07ab cde fgh',
+    ],
+    'valid': [
+        '07123456789',
+        '07123 456789',
+        '07123-456-789',
+        '00447123456789',
+        '00 44 7123456789',
+        '+447123456789',
+        '+44 7123 456 789',
+        '+44 (0)7123 456 789'
+    ]
+}
+
+
+@pytest.mark.parametrize("phone_number", phone_numbers['valid'])
+def test_phone_number_accepts_valid_values(phone_number):
+    form = FormExample(phone_number=phone_number)
+    form.validate()
+    assert form.errors == {}
+
+
+@pytest.mark.parametrize("phone_number", phone_numbers['invalid'])
+def test_phone_number_rejects_invalid_values(phone_number):
+    form = FormExample(phone_number=phone_number)
+    form.validate()
+    print(phone_number)
+    assert form.errors != {}
+
+
+@pytest.mark.parametrize("phone_number", phone_numbers['valid'])
+def test_phone_number_outputs_in_correct_format(phone_number):
+    form = FormExample(phone_number=phone_number)
+    form.validate()
+    assert form.phone_number.data == '+44 7123 456 789'