The move, revealed yesterday by Paddy Ashdown, the Liberal Democrat leader, after briefings from US sources, threatens to undermine Trident as an independent nuclear deterrent.
It could also put further strain on the delicate "special relationship" between London and Washington, which has been under threat since President Bill Clinton's election. The US provides all the servicing for the Trident system, and the UK would find it very difficult to resist firm pressure from Washington.
The US and Russia are anxious to extend the Nuclear Non-Proliferation Treaty. But some countries are saying that Britain and other traditional nuclear powers are not doing enough to dismantle their weapons.
Mr Ashdown, a senior Privy Councillor and defence specialist, is due to leave tomorrow for a meeting with the US Vice-President, Al Gore, in Washington. He learned of the American position on Trident during preparatory briefings with US officials and politicians.
Mr Ashdown said: "This is America the protector about to become the whip- cracker. This will be harsh reality for those who have always vested our defence and security interests in America. It will also underline the increasing urgency with which we, in Britain, need to consider integrated European defence security measures."
Britain and France have, so far, remained outside the scope of US/Russian arms reduction treaties. But, now that the larger blocs have decommissioned huge numbers of weapons, the British and French nuclear forces have become statistically more important. Britain has four Trident submarines and the Ministry of Defence is committed to deploying no more than 96 warheads on each boat - regarded by the UK as the minimum credible deterrent.
The Nuclear Non-Proliferation Treaty will be discussed in April, with the big powers aiming for an indefinite extension but other countries demanding more action from the established nuclear powers.
The Russians are increasingly concerned about the extension of Nato eastwards, arguing that British and French nuclear weapons reductions should counterbalance Nato enlargement.
One possibility is a new Strategic Arms Reduction Treaty, or a similar agreement, to be negotiated in the wake of the Non-Proliferation Treaty. Foreign Office sources played down the significance of Mr Ashdown's contacts, arguing that different people in the US administration held different views. The Americans had made no formal request for reductions, they said.
But sources in the US who are closely monitoring the nuclear disarmament process said it came as no surprise that the Russians and the Americans should be putting pressure on Britain and France to cut back their nuclear capability.
"At a time when radical bilateral disarmament is under way, when America and Russia are working in good faith to achieve the objectives agreed in the Nuclear Non-Proliferation Treaty, Britain and France have both signalled they have no intention of getting rid of their nuclear weapons," one official said.
"We understand that Douglas Hurd has told a parliamentary committee that he does not foresee a day when Britain will not have nuclear weapons. Indeed, what we see is that while America and Russia are moving to cut the number of warheads each has from the tens of thousands they originally had to 6,500 by the year 2003, Britain has just modernised its system through Trident."
Sooner or later, say those involved in disarmament negotiations, Britain, France and China, the three smaller nuclear powers, will have to follow the lead of the big two.
Menzies Campbell, the Liberal Democrat foreign affairs spokesman, who is also visiting the US, said that Trident, with about twice the range of Polaris, "represents a considerable enhancement of nuclear capacity".Reuse content