Gordon Brown looks likely to face a leadership challenge from the left of the Labour Party tonight, after John McDonnell confirmed he would attempt to gather the necessary nominations to stand against him.

The backbencher announced he would try to challenge the Chancellor after pooling his support with the fellow leftwinger Michael Meacher, who withdrew his candidacy.

The move - which took place on the day that the Labour leadership nominations officially opened - dramatically improves the left's chances of gaining the 45 nominations necessary to register a candidate in the race and prevent the "coronation" of Mr Brown without a contest.

At a press conference held this evening at a Westminster meeting room Mr Meacher, the former Environment Secretary, explained that he had withdrawn because his colleague had accumulated a larger list of backers.

He appealed to all the MPs who initially pledged their support to him to now back Mr McDonnell in order to give Labour members and trade unions around the country a chance to have their say.

"I am asking all of my supporters to get behind John," he said, adding that he wanted a "united left platform for the leadership".

Mr McDonnell added that he was "confident" that the combined pledges of support gathered by the two men would pass the threshold of 45, but stressed that he required the support of a large number of his opponent's backers.

"What we are now doing is we are launching an appeal to Labour MPs," he said. "This is an issue of democracy."

Mr McDonnell now has until Thursday at 12.30pm, when nominations close, to secure the necessary support to stand against Mr Brown.

It would have been impossible for both men to have raised the minimum number of MPs to stand without pooling their resources, as Mr Brown has gained the overwhelming backing of his party and is the huge favourite. The Chancellor has previously said he would welcome a contest.

On a visit to Southampton today, Mr Brown continued to set out his stall, pledging to increase by 50 per cent the building of social housing. The Chancellor last weekend has also proposed building five 'eco-towns,' a pledge dismissed by the Conservatives as an old Labour policy which had never been implemented.

"It’s been a pleasure to work with Tony Blair as Prime Minister over the past 10 years, but, while we have climbed one mountain, there are other mountains to climb," he said, in a speech at a housing development in the town.

In the battle for the deputy leadership, nominations for which also officially opened today, six candidates have so far put their names forward.

Hazel Blears, the Labour chairman; Harriet Harman, the Justice Minister; and Peter Hain, the Northern Ireland Secretary, have all announced that they have the nominations necessary to stand while Alan Johnson, the Education Secretary, is believed to have more than the necessary backers.

John Cruddas, a left-wing backbencher, and Hillary Benn, the International Development Secretary, have yet to make a formal announcement, however they say they are "confident" of having enough nominations to stand.

Speaking to a gathering of about 40 backers in Westminster this afternoon, Ms Harman announced she had 61 pledges of support. Prominent supporters include the Transport Secretary Douglas Alexander and Alistair Darling, the Trade and Industry Secretary.

"We’ve got work to do, and our task could hardly be more important," she said. "It’s nothing less than renewing the Labour Party and rebuilding the full trust and confidence of the British people in this Labour Government."

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