Ex-Cabinet minister Clare Short has warned Gordon Brown his support for new UK nuclear weapons could damage his chances of becoming prime minister.
Ms Short said it was "outrageous" Mr Brown had signalled that he wanted to replace the Trident weapons system.
She said she and other Labour MPs would not vote for him in a leadership contest unless he changed his mind.
Downing Street said there would be a "full debate" on Trident's future but did not promise MPs a vote.
1980 Margaret Thatcher commits Britain to having Trident
1993 Trident submarine-based nuclear missile programme comes into use replacing Polaris
Late 2006/early 2007 Cabinet decision on replacing Trident expected
Around 2010 work on new scheme to replace Trident is expected to begin
2024 Britain's Trident submarines due to be decommissioned
But Education Secretary Alan Johnson later suggested he might like to see a vote, although he stressed no decisions had been taken.
There had been a vote on the Iraq war for the first time and "perhaps on this issue as well for the first time ever we will have a debate and a decision on the nuclear deterrent", he told BBC One's Question Time, to be screened at 2245 BST.
The submarine-based Trident missile system will become obsolete by 2024. A replacement could cost up to £25bn.
Mr Brown, who wants to be the next prime minister, has faced a backlash since using his Mansion House speech in the City of London to voice his personal commitment to keeping the UK's "independent nuclear deterrent".
Speaking on BBC Radio 4's World At One, Ms Short accused Mr Brown of showing disrespect for "any kind of democratic process".
"It's part of his desperation that's so humiliating him to prove to the Blairites that he's as right-wing as Blair and therefore that they will keep their promise to hand over to him," she said.
October 1952 Britain tests nuclear weapon
May 1957 First UK hydrogen bomb detonated
1958 US-UK Mutual Defence Agreement signed allowing the sharing of nuclear technology
1968 British submarine armed with US-built Polaris missiles goes on patrol for the first time
November 1968 UK ratifies nuclear non-proliferation treaty
1982 Cruise missiles deployed in UK
Ms Short added: "It means a lot of people who were happy to see Brown take over as leader will now think there's got to be a contest and we're not willing to support him."
Some delegates at public sector union Unison's conference suggested backing Trident could lose Labour the next election.
The 1,700 delegates voted unanimously to oppose replacing Trident. They also said the existing weapons system should be scrapped.
Preparations under way
A decision on Trident is expected to be taken in months rather than years.
Defence Minister Adam Ingram told MPs: "No decisions have yet been taken in principle or detail on any replacement for Trident".
Mr Ingram said a decision was needed before the next election and officials were working on "risks, threats, options and costs to prepare the ground for eventual decisions by ministers".
Former Tory foreign secretary Malcolm Rifkind branded Mr Brown's remarks "shabby".
"It is not the responsibility of the chancellor in an after dinner speech to announce nuclear policy," he said.
The issue of Britain's independent nuclear weapons system has always been controversial, especially within the Labour Party, which opposed Margaret Thatcher's decision to commission Trident in the 1980s.
But in his speech, Mr Brown said: "In an insecure world we must and we will always have the strength to take all necessary long term decisions to ensure both stability and security."
Kate Hudson, chairwoman of the Campaign for Nuclear Disarmament, warned: "At this point, when we face no nuclear threat, to decide on a new Trident replacement is beginning a new nuclear arms race," she said.
Labour backbencher Gordon Prentice asked: "How are we going to persuade other countries not to go for nuclear weapons when we are spending millions of pounds not disarming but upgrading our nuclear weapons?"
The Conservatives say they too are committed to replacing Trident to avoid leaving the UK "exposed and vulnerable".
But shadow defence secretary Liam Fox described the government's handling of the issue as "disgraceful".
"What we are getting is power politics within the Cabinet rather than the national interest being put first," he said.
Liberal Democrat defence spokesman Nick Harvey said: "Gordon Brown's posturing on Trident is smothering the national debate that this government promised to the British people."