David Miliband says he is not planning to run for Labour's deputy leadership - but Northern Ireland and Wales Secretary Peter Hain is to go for it.
Mr Miliband is talking to the unions about climate change
Mr Miliband told BBC News: "I am not a runner nor a rider for any of the jobs that are being speculated about."
Meanwhile Mr Hain confirmed he would stand for the deputy leadership "when that contest happens next year".
Deputy Prime Minister John Prescott is expected to quit his job at the same time as the prime minister.
Both Mr Hain and Mr Miliband, who was one of a number of Cabinet ministers seen as a possible contender, say Gordon Brown should be the next leader.
Mr Hain has reportedly been wooing the trade unions, which will have one third of the votes in the deputy leadership contest.
Peter Hain has apparently been courting union chiefs
Announcing his candidature, he said: "I will be seeking that office not for its own sake but in order to take forward the process of party reform and renewal."
Mr Hain said there was no "ideological schism" inside Labour but renewing the party in government would be difficult.
"We need to reconnect the leadership with the grass roots in the party and we need to reconnect the government to civil society and people and citizens outside," he said.
"I have been very concerned and have spoken out on occasion about the disconnection there has been between policies often handed down from on high."
Labour's senior command's communications with party members often felt like "a lecture rather than a dialogue", he said.
Mr Hain pointed to four policy areas he wanted to address:
- A new drive on social justice
- A revitalised commitment to decentralising power and reinvigorating local government so Labour was seen as the party of "personal empowerment and individual liberty"
- Finishing reform of the House of Lords and introducing the alternative vote system for elections
- Ensuring "green" government.
Mr Hain also said a debate was needed on the limits of involving the private sector in running public services - a key flashpoint at this week's TUC conference.
These changes could reassemble the coalition which brought Labour to power in 1997, he said, admitting the party had lost large parts of that alliance in last year's poll.
Mr Hain said he hoped to be a "bridge" to the future for New Labour. He said the party must not turn its back on its success - saying that had been the mistake made by Al Gore when he fought George W Bush in the US presidential elections.
Earlier, Mr Miliband used his platform speech at the TUC to argue that trade unions have a key role to play in tackling climate change.
He said: "The trade unions were absolutely central to economic and social improvement in the 20th Century.
"My message to them today is they can be at the heart of environmental improvement in the 21st."
The environment secretary faced questions about the effect of rising fuel prices for those living in poverty.
One delegate blamed the privatisation of UK energy companies, asking whether markets were really the right way to ensure energy stability.
Mr Miliband countered that oil prices were not set in the UK but were a global issue.
The high prices made renewable sources of energy more competitive, he said, but the obvious down side was that they made life more expensive, especially for those in fuel poverty.
He said the answer was to increase energy efficiency programmes. And the winter fuel allowance for pensioners, introduced in 1998, was now coming into its own.