Former British environment minister Michael Meacher has announced he will challenge Gordon Brown for the Labour leadership.
Mr Meacher had been widely expected to put his name forward as a left-wing candidate to succeed British Prime Minister Tony Blair.
Last November he hinted strongly that he would stand against Mr Brown after Mr Blair quit, and said climate change should be at the heart of any prospective leader's campaign.
Mr Meacher (67) is one of the party's longest serving MPs, having first been elected to Parliament in 1970.
He served in various capacities in the Wilson and Callaghan governments, and during Labour's long period in opposition, he served continuously in the shadow cabinet
Mr Meacher was the left's candidate against Roy Hattersley for the deputy leadership in 1983 but was soundly defeated.
PA