The London Assembly is a 25-member elected body, part of the Greater London Authority, that scrutinises the activities of the Mayor of London and has the power, with a two-thirds supermajority, to amend the Mayor's annual budget and to reject the Mayor's draft statutory strategies. The London Assembly was established in 2000. It is also able to investigate other issues of importance to Londoners, publish its findings and recommendations, and make proposals to the Mayor.

The Greater London Authority (GLA), colloquially known by the metonym City Hall, is the devolved regional governance body of Greater London, England. It consists of two political branches: an executive Mayor and the 25-member London Assembly, which serves as a means of checks and balances on the Mayor. Since May 2016, both branches have been under the control of the London Labour Party. The authority was established in 2000, following a local referendum, and derives most of its powers from the Greater London Authority Act 1999 and the Greater London Authority Act 2007.