What is the capital of London?

The question is asked often enough to deserve a straight answer rather than a correction. London is a city. Cities do not have capitals; countries, states and regions do. What London has is a seat of its own government, and three different things are called London, so it is worth being clear about which one is meant.

Three Londons

Greater London is the administrative region: 32 boroughs plus the City of London, about 8.9 million people. Its government is the Greater London Authority, meaning the Mayor of London and the London Assembly, and since 2022 it has worked from City Hall at the Royal Docks in Newham.

The City of London is the square mile at the centre. It is a local authority of its own with its own Lord Mayor and its own police force, and it is one of the 33 authorities inside Greater London, not the capital of any of them.

London the built-up area spills past the regional boundary into Essex, Hertfordshire, Surrey and Kent, and has no government of its own at all.

The closest thing to an answer

If the question means where London governs itself from, that is City Hall at the Royal Docks in Newham, the home of the Mayor of London and the London Assembly since 2022. Before that it was on the South Bank, opposite the Tower, and before the Greater London Authority existed at all the London County Council sat at County Hall on the same stretch of river.

None of those buildings is a capital. A capital is a city, and City Hall is an office.

Where London is the capital

Westminster has been the seat of English government since the eleventh century, and of the United Kingdom's parliament since the union of 1707. England has no devolved parliament, so the Westminster parliament legislates for England directly and London is England's capital by the same act that makes it the United Kingdom's.

See London and the United Kingdom for the records behind this, and what is the capital of England? for the part that trips people up most.