Left-wing lawmaker John McDonnell joined the race to lead Britain's Labour party on Wednesday, calling on trade unions and party members to support his bid to return Labour to its roots.
The centre-left party is searching for a new leader to succeed former Prime Minister Gordon Brown, who resigned after the party lost its 13-year grip on power in the May 6 election.
A Conservative-Liberal Democrat alliance now governs Britain, the first coalition since World War Two.
Brown's predecessor, Tony Blair, moved the traditionally working-class party to the centre in the 1990s, calling it "New Labour" and winning three elections in a row, but this month's defeat has left the party searching for a new direction.
"I believe (New Labour) lost the moral basis of the Labour Party as it was founded ... the creation of a fair and just and peaceful and equal society," McDonnell said in a rousing speech to the Public and Commercial Services union.
"Join me in that campaign to advance what I define, what you define, as socialism," he added.
McDonnell, 58, who is not well known outside the party, is viewed as an outsider in the leadership stakes, but has support among the unions, Labour's biggest financial backers.
He tried to challenge Brown for the Labour leadership in 2007 when Blair stepped down, but failed to win enough support to force a vote.
McDonnell's entry into the race will broaden the debate and mean party members are offered a left-wing alternative to the more centrist candidates who have so far come forward.
Former schools minister, Ed Balls, 42, a close Brown confidant who is seen as left-leaning, is also expected to announce his candidacy on Wednesday, according to the Guardian newspaper.
The frontrunner is the cerebral David Miliband, 44, foreign minister under Brown. Once an adviser to Blair, he is seen as the candidate of the party's "Blairite" or centrist wing.
The only other candidate to come forward so far is his brother Ed Miliband, 40, a former energy and climate change minister. His supporters say he is a unity candidate who would end years of tension between the so-called "Blairite" and "Brownite" wings of the party.
Labour has given itself four months to elect a new leader. Balloting will run from Aug. 16 to Sept. 22 and the winner will be announced at the annual party conference on Sept. 25.