If you are a British or Irish citizen flying within the Common Travel Area (which includes the UK and the Republic of Ireland), you do not legally require a passport to fly to Northern Ireland. However, you are strictly required to show a valid form of photo identification to board your flight. Most airlines, such as British Airways, easyJet, and Loganair, accept documents like a valid photo driving license, a citizen card, or a government-issued employee ID. It is important to check the specific requirements of your airline, as some carriers have stricter internal policies than the government’s legal minimum. If you are not a British or Irish citizen, you will almost certainly need a valid passport to satisfy immigration requirements, even on a domestic flight from London to Belfast. Additionally, while you may not need a passport for the flight itself, having one is the most universally accepted form of ID and can prevent any potential boarding delays. Always ensure your chosen ID is in good condition and matches the name on your boarding pass exactly to ensure a smooth transition through the security and boarding gates at airports like Belfast International or George Best Belfast City.