Alan Johnson has launched his bid for the Labour deputy leadership with an open letter to party members asking for their support.
The Education Secretary had been widely tipped as a potential rival to Gordon Brown for the leadership, but has ruled out a bid for the top job, saying the Chancellor was a "vastly superior" candidate.
Mr Johnson will kick off his campaign with a speech to his constituency party in Hull, telling them that Labour must occupy the political centre ground and appeal to voters from all parts of society.
He will set out plans for a 24-month campaign to retain the key marginal seats which will decide the next General Election, promising that as deputy he would "start the process of winning the fourth term straight away".
In his open letter, he wrote: "I am not putting myself forward for Leader - not least because there's a more experienced and obvious candidate.
"But I will be able to assist, support, cajole and complement the person who carries the heaviest of all political burdens."
The deputy must be "subordinate to and supportive of the leader, (carrying) out whatever duties the leader sees as being essential to securing a fourth term in office".
Mr Johnson, 56, joins a crowded field in the race to succeed John Prescott, with Cabinet ministers Peter Hain and Hilary Benn, constitutional affairs minister Harriet Harman and left-wing backbencher Jon Cruddas already declared, and Leader of the Commons Jack Straw also expected to stand.
A former leader of the Communication Workers Union, Mr Johnson said he would fight to preserve Labour's link with the unions, without which he said working-class people would have "little voice or influence in our society". But he warned against a return to the left-wing "dogma" which consigned Labour to opposition in the 1980s.
