Gordon Brown has signalled that he wants to keep and renew Britain's independent nuclear deterrent.
The Trident missile system and the Vanguard submarines which carry them need replacing by 2024 and a decision is set to be taken in the next year.
Estimates of the cost vary from £10bn to £25bn, depending on what type of new missiles or submarines are chosen.
Mr Brown's intervention has enraged critics, who say Trident has no use now the Soviet Cold War threat is over.
Labour had a manifesto commitment to retain an independent nuclear deterrent but it only applies until the next general election.
Mr Brown, seen as the most likely next prime minister, has sparked new debate on the issue by highlighting his personal commitment to replace Trident.
In his Mansion House speech in the City of London, He said Britain would show a "national purpose" in protecting its security.
"Strong in defence in fighting terrorism, upholding NATO, supporting our armed forces at home and abroad, and retaining our independent nuclear deterrent," he said.
"In an insecure would we must and we will always have the strength to take all necessary long term decisions to ensure both stability and security."
'No moral reason'
It is thought Mr Brown wants anti-nuclear campaigners to know he is just as committed to replacing Trident as Tony Blair.
BBC political editor Nick Robinson said Mr Brown's words would take the heat off the prime minister, who could have produced "uproar" if he had made the same announcement.
The government's position is that decisions on updating or replacing Trident are likely to be needed during the current Parliament.
A Ministry of Defence spokesman said: "No decisions have been taken on the replacement of Trident, either in principle or detail."
But the decision is expected to be taken in months rather than years.
Anti-nuclear groups, Labour backbenchers and trade unionists voiced their alarm at Mr Brown's words.
Kate Hudson, chairwoman of the Campaign for Nuclear Disarmament, said: "We were hoping that any potential future prime minister would stick by the commitments made last year by then Defence Secretary John Reid for a full public and parliamentary debate.
"Our feeling is statements like this from someone as significant as Gordon Brown pre-empts that debate."
Ms Hudson said this was the moment to start multi-lateral disarmament talks.
"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 MP Ian Gibson, an opponent of Trident, said many young Labour backbenchers had been weaned on CND and had not lost those early political views.
"So it may not be as easy [to agree to replace Trident] as people might think because the chancellor says so," he told BBC News 24.
Another 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?"
Keith Sonnet, deputy general secretary of Unison, the country's biggest trade union, also urged Mr Brown to think again.
The Conservatives accused Mr Brown of "spin" designed to make him look statesmanlike when he was in fact just repeating Labour's 2005 manifesto.
Shadow defence secretary Liam Fox said: "The chancellor is reheating an old pledge to retain the current nuclear deterrent but he is not committing to replacing the independent nuclear deterrent when it reaches the end of its current life."
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," he said.
Earlier, Tony Blair promised the "fullest possible debate" on Trident, but stopped short of promising a vote.