Government officials said the move was not aimed at cutting costs but to show leadership on nuclear non-proliferation.
"If we are serious about the ambition of a nuclear-free world we will need statesmanship, not brinkmanship," Prime Minister Gordon Brown will tell the United Nations, according to an aide.
Brown flew into New York on Tuesday to attend the U.N. General Assembly and will travel to Pittsburgh for a summit of G20 leaders to discuss the next steps in pulling the world out its worst recession since the 1930s.
He will tell the U.N. Security Council on Thursday that Britain is not ready to give up its status as a nuclear power but is prepared to reduce its nuclear submarine fleet when it is replaced over the next 15 years.
The present Vanguard class of submarines is likely to start leaving service from the early 2020s and plans call for its successor to be in service by 2024. Likely suppliers for the new submarines are BAE Systems and Rolls-Royce.
There is no automatic link between reducing the number of submarines and reducing Britain's 160 warheads, Brown will maintain.
Britain is expected to spend around 20 billion pounds replacing its Trident nuclear deterrent and there has been much speculation this is an area of expenditure that could be reduced in order to rein in a burgeoning budget deficit.
The deficit is expected to top 12 percent of GDP and how to reduce government spending has become a key political battleground ahead of an election that must take place by June.
Officials said that there would not be much saved in cutting the number of submarines because the capabilities of the reduced fleet would have to be enhanced for "the three boat solution."
The Conservative Party, widely tipped to win an election due by June, has also signalled it is ready to reconsider the scale of the Trident programme.
Officials said the next stage in the process would be a review by a government committee by the end of the year.
Meeting Gaddafi?
Brown will tell the United Nations on Wednesday that, in addition to nuclear proliferation, the world faces four major challenges -- climate change, the global economy, terrorism and poverty.
"A safer Afghanistan means a safer world. But none of us can be safe if we walk away from that country or from our common mission and resolve," he will say.
On the global economy, Brown is expected to back making the G20 the central decision-making forum for the world economy
Thursday's U.N. Security Council meeting also raises the prospect of Brown meeting Libyan leader Muammar Gaddafi for the first time since the release of Lockerbie bomber Abdel Basset al-Megrahi from a Scottish prison on health grounds.
Megrahi had been in prison for his role in the 1988 bombing of a U.S. airliner over Scotland in which 270 people died but was given a hero's welcome on his return to Tripoli, which raised public anger in Britain and the United States.
