John McDonnell says he wants candidates to stay within the Labour Party
Left wing Labour MPs are threatening to stand under their own manifesto at the next election if Prime Minister Gordon Brown fails to adopt their policies.
John McDonnell, chair of the Campaign group of Labour MPs, stresses he wants candidates to remain in the party.
But he said they could stand as Labour "change candidates" if Gordon Brown fails to alter his political course.
It comes as Blairite MP Charles Clarke said there could be a leadership challenge before the next election.
Both the the left and right wings of the party appear to be suggesting that Mr Brown is on probation after Monday's meeting of Labour MPs in which he promised a more inclusive style of government.
Mr McDonnell, who failed to gain enough support to challenge Mr Brown before he entered Downing Street in 2007, said the Labour left had deliberately steered clear of recent plots to oust Mr Brown and wanted to see a change of political direction instead.
Labour candidates standing on their own policy platform would be seen as a highly provocative move by the party leadership and could potentially see them being expelled from the party.
Mr McDonnell is careful to stress that he wants candidates to "bind together as a slate" within the Labour Party and set out to voters the policy programme they would advocate and vote for in Parliament.
But a Labour Party spokesman said: "Policy in the Labour Party is made via Partnership in Power, an inclusive and consultative process unique in British politics.
"Through discussion with the public, ordinary Labour members from each region and nation of Britain, elected members, and representatives from Labour Party's affiliated organisations come together to agree a policy platform which best matches the aspirations and concerns of the British people.
"This process involves as many people as possible and not just a tiny section of the party or indeed the country."
Among the policies Mr McDonnell wants to see is the restoration of trade union rights, more council houses, an end to public service privatisation, scrapping the Heathrow third runway, freezing and then abolishing student fees, scrapping Trident and ID cards and electoral reform.
He said: "If Labour is to stand any chance of surviving at the next election real change has to be visibly underway and progress demonstrated at the latest by the autumn. If we go beyond November without real change, what hope is left of Labour not only remaining in government but surviving as an effective political force at all?"
He said a leadership challenge at that stage "would almost certainly be blocked again by MPs failing to nominate" and the only alternative would be for Labour MPs to "stand on a policy platform of real change as 'change candidates'" at the next general election.
Clarke speaks out
Mr McDonnell chairs the Socialist Campaign Group, which includes 24 Labour MPs.
They represent some of Labour's safest seats but some, such as Frank Cook and Bob Marshall-Andrews, are standing down at the next election and Ian Gibson is quitting to force a by-election, after being barred from standing as a Labour candidate after criticism of his expenses claims.
Meanwhile, at the other end of Labour's political spectrum, former home secretary Charles Clarke has said he still wants Mr Brown to stand down but he admits that a recent attempt by some backbenchers to remove him is now over.
But he said it "depends entirely on Gordon" as to whether he led the party into the next election.
In an interview with Andrew Neil's Straight Talk, to be broadcast on Saturday, June 12 at 2230 on the BBC News Channel, he said: " "If, for example, the poll ratings go up or we win these by-elections which are going to come through or whatever, I think the issue will go away and he can be confident he leads us into the next election."
But he added: "If, on the other hand, he somehow doesn't fulfil those things or electorally we do badly or whatever it might be, then the issue will still be there.
He added: "He himself gave a set of commitments to the Parliamentary Labour Party at that meeting last Monday night about his performance, his behaviour, his approach, and I think people will look at all those things and make their judgements in the light of them."