Mayor of London: Ken slams Boris in opening salvo of mayoral election

The Labour veteran – who was ousted by the flamboyant Tory in 2008 after eight years – was in Croydon to open his campaign to be Labour's candidate in 2012.

He said his priority would be to "protect" Londoners from the recession and Government spending cuts and promised to freeze his own and senior advisers' pay for the full term.

Mr Livingstone formally entered the contest just days after ex-MP Oona King announced her intention to fight him for the nomination as "a breath of fresh air" for the capital.

In a thinly-veiled attack on her older rival, Ms King warned Londoners did not need a "popularity contest based on who's whacky or who's stale".

But Mr Livingstone reserved his fire for the incumbent mayor and released a list of high-profile Labour politicians backing him.

"The global economic crisis, a fragile recovery that may go double-dip, and a Government removing billions from the economy and planning cuts on a scale that Britain has not seen for decades, mean the mayor's priority must be to protect Londoners," he said.

Hopefuls have until June 18 to put their names forward. An electoral college, made up half-and-half of votes from London party members and members of affiliated organisations, will then pick the candidate.

The result will be announced on September 22 – the day before Labour reveals its new party leader.