UK phone number format — E.164, regex, rules

If you are building software, cleansing a CRM, or running a UK ad campaign with a click-to-call button, you need a robust definition of a valid UK phone number. This guide is the engineering-grade reference: the E.164 spec, the libphonenumber-style regex, the prefix table, and a working JavaScript validator.

#UK phone number lengths

#UK phone number prefixes

#UK phone number regex (and why you probably should not write your own)

A good-enough UK regex for E.164:

^\+44(?:\d{9,10})$

And for national format:

^0(?:1\d{8,9}|2\d{9}|3\d{9}|5\d{9}|7\d{9}|8\d{9}|9\d{9})$

#Validating with libphonenumber-js

The right way, in TypeScript:

import { parsePhoneNumberFromString } from "libphonenumber-js/min";

export function isValidUkNumber(input: string): boolean {
  const phone = parsePhoneNumberFromString(input, "GB");
  return Boolean(phone && phone.isValid() && phone.country === "GB");
}

export function toE164(input: string): string | null {
  const phone = parsePhoneNumberFromString(input, "GB");
  return phone && phone.isValid() ? phone.number : null;
}

export function toNational(input: string): string | null {
  const phone = parsePhoneNumberFromString(input, "GB");
  return phone && phone.isValid() ? phone.formatNational() : null;
}

The default country GB tells libphonenumber to assume UK national format when there is no leading +. The library handles every edge case in the table above plus a few you have not thought of (e.g. numbers with formatting characters from CSV exports).

#E.164 specifically - what to store in your database

Store every UK number as E.164 (+44...). It is:

• Globally unambiguous - +442079460000 is a London number anywhere on Earth.
• Round-trippable - parsePhoneNumberFromString('+442079460000', 'GB').formatNational() gets you back to display format.
• Index-friendly - fixed alphabet, no formatting variants to deduplicate.
• Ready for SMS APIs - Twilio, Vonage, MessageBird all want E.164.

Display in national format on UI; store in E.164 in the database. Never the reverse.

#International phone-number checker - UK and beyond

If you need to validate non-UK numbers in the same code path, libphonenumber-js covers 240+ countries out of the box. For server-side bulk validation at scale, a paid API like Twilio Lookup gives you carrier and line-type data on top of the format check - see UK phone number validation API for the comparison.

#Bottom line

UK phone number format is well-defined but full of edge cases. Use libphonenumber-js for validation, store E.164 in your database, display national format on UI. The snippet above is sufficient for 99% of consumer apps; for bulk B2B work, see the API and CRM-cleansing guides linked above.