Britain's new government revealed Wednesday the planned size of its nuclear weapons stockpile, saying it will not exceed 225 warheads -- a move cautiously welcomed by anti-nuclear campaigners.

In an announcement coinciding with the end of a United Nations nuclear non-proliferation treaty conference in New York, Britain said it will retain up to 160 operationally available warheads.

"We believe that the time is now right to be more open about the weapons we hold," Foreign Secretary William Hague told parliament.

"We judge that this will assist in building a climate of trust between nuclear and non-nuclear weapons states and contribute therefore to future efforts to reduce the number of nuclear weapons worldwide."

The statement makes public for the first time the maximum number of warheads Britain will stockpile.

Hague said the move reflected the new coalition government's commitment to play a full part in the UN conference, which is due to close on Friday.

He said the announcement posed no threat to British security, with the United States and France having made similar announcements -- Paris said in 2008 that its arsenal will include fewer than 300 nuclear warheads, while the US announced earlier this month that it has 5,113 warheads, said the FCO.

The Campaign for Nuclear Disarmament (CND) cautiously welcomed the announcement.

"Publishing this number is a welcome step that can help build trust between states and open the way for disarmament," said CND chief Kate Hudson. "But Britain's commitment under the NPT is not to be transparent, but to disarm.

The number "gives a baseline against which further disarmament can be measured, but it is the reduction and then elimination of the UK and other countries' nuclear weapons that will deliver real security," she added.

Britain's nuclear deterrent is one of the points where Britain's governing coalition parties, the Conservatives and the smaller Liberal Democrats, differ drastically.

The Conservatives believe the Trident missile programme must be renewed, while the Lib Dems think it is outdated and too costly.

The British minister said the government would re-examine the circumstances under which Britain might consider using its nuclear weapons.

"The purpose of our nuclear weapons is to deter attack and the United Kingdom has long been clear that we would only consider using them in extreme circumstances of self-defence, including the defence of our NATO allies.

"This country has been deliberately ambiguous over the precise circumstances of use, although we have offered some assurances to non-nuclear weapons states," he said.

"However we have decided that the time is right to look again at our policy, as the US has done in their recent nuclear posture review, to ensure that it is fully appropriate for the political and security context in 2010 and beyond."

CND chief Hudson said: "The figures reveal that Britain has almost half as many warheads again as the previously published number of 'operationally available' -- higher than many had estimated.

"This very large number of 'spares' kept by Britain is similar to the supposed size of the entire nuclear arsenals of India or Pakistan," she added.