British Prime Minister Gordon Brown's ruling Labour Party on Friday suffered its worst defeat in local elections in four decades while its candidate for London [Images] mayor Ken Livingstone lost his bid for a third term.

In a tight contest, Labour candidate Livingstone lost to Conservative Party challenger Boris Johnson, official sources said.

The Conservatives grabbed 44 per cent of the vote, with the Liberal Democrats winning 25 per cent, pushing the Labour Party to the third position.

In the polling held on Thursday, the ruling Labour lost as many as 400 council seats, mostly to Conservatives and Liberal Democrats.

The outcome is an embarrassment for Brown, who took over as prime minister from Tony Blair [Images] in June 2007.

He now faces an uphill political battle against the Conservatives, if he is to remain in office following nationwide parliamentary elections that is due to be held early 2010.

Livingstone, the left-wing Labour Party incumbent nicknamed "Red Ken" was credited with his response to the July 7, 2005, bombings and for bringing the 2012 Olympics [Images] to London.

Johnson, a former newspaper columnist and TV quiz show host, is a member of Parliament.

Prime Minister Gordon Brown told newsmen "this has been a disappointing night -- indeed, a bad night for Labour."