Children under the age of 16 years do not require identification to travel within the UK. All other travellers require a valid passport to travel between the two countries.

Whether you need a passport for Cardiff, Wales, depends entirely on your nationality and where you are traveling from. If you are a British or Irish citizen traveling from within the UK or the Republic of Ireland (the Common Travel Area), you do not legally need a passport to enter Wales, though you should carry a form of government-issued photo ID (like a driver's license) for airline or hotel checks. However, as of February 25, 2026, the UK has fully enforced its Electronic Travel Authorisation (ETA) system. If you are a "non-visa national" (such as a citizen of the U.S., EU, or Australia) traveling to Cardiff from outside the UK, you must have a valid passport and a pre-approved ETA. Dual British citizens are now strictly required to use their British passport to enter. If you are already within the UK (e.g., traveling from London to Cardiff by train), there are no border controls or passport checks whatsoever.