FACTBOX - Local elections

(Reuters) - Voters elect the mayor of London and 4,023 local councillors in England and Wales on Thursday in Gordon Brown's first electoral test since taking over as prime minister from Tony Blair last June.

Here are some key facts and figures. Voting will run from 7 a.m. to 10 p.m.

WHO NEEDS WHAT

Labour Party - If Labour loses seats, it will have had its worst set of local election results since the days of Harold Wilson back in 1968. The party could probably live with up to 100 losses given the economic climate, but anything approaching 200 would be seen as worrying for Brown's parliamentary election prospects. A parliamentary election must be held by mid-2010.

A win for Labour's Ken Livingstone in the London race would be a big boost to party morale, but he faces a strong challenge from Conservative candidate Boris Johnson.

Conservative Party - Again the key figure is 200. A gain of more than 200 seats would confirm the Conservatives' opinion poll lead. Between 100 and 200 seats would be good but not great and any fewer than 100 would suggest they still have to convince the electorate they are ready to form the next government.

Liberal Democrat Party - Last year Britain's third party lost about 250 seats in England and this time analysts expect it to be hoping for around 50 gains. Any fewer would be a poor start for new leader Nick Clegg.

KEY SEATS

Reading is the last Labour local stronghold in the southeast of England and a swing of 4 percent to the Conservatives would see Labour lose overall control for the first time since 1986. A similar swing would see Labour lose Nuneaton and Bedworth in central England.