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."