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The administration of London is formed of two tiers—a city-wide, strategic tier and a local tier. City-wide administration is coordinated by the Greater London Authority (GLA), while local administration is carried out by 33 smaller authorities. The GLA consists of two elected components; the Mayor of London, who has executive powers, and the London Assembly, which scrutinises the mayor's decisions and can accept or reject the mayor's budget proposals each year. The headquarters of the GLA is City Hall, Southwark; the mayor is Boris Johnson. The mayor's statutory planning strategy is published as the London Plan, which was most recently revised in 2011. The local authorities are the councils of the 32 London boroughs and the City of London Corporation. They are responsible for most local services, such as local planning, schools, social services, local roads and refuse collection. Certain functions, such as waste management, are provided through joint arrangements. In 2009–2010 the combined revenue expenditure by London councils and the GLA amounted to just over £22 billion (£14.7 billion for the boroughs and £7.4 billion for the GLA).

London's government administration is comprised of how many tiers?
two