You must have a valid passport to enter the UK. It should be valid for the whole of your stay. You may also need a visa, depending on which country you're from.
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Your identity document (for example your passport or identity card) will be checked when you arrive at a UK port or airport to make sure you're allowed to come into the country. It should be valid for the whole of your stay. You may also need a visa to come into or travel through the UK, depending on your nationality.
If you are passing through a UK airport while on the way to another country, you may need a UK transit visa. Transit visas are required by some nationals who are passing through the UK en-route to another country within 24 or 48 hours.
Generally, it is necessary to have a valid passport if you want to travel abroad. Depending on the country and your citizenship, you may also need to show proof of a valid eVisa, online visa, or visa.