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UK says could cut warheads if US, Russia go further

* British government still wants nuclear deterrent

* Ready to join talks if U.S., Russia decide on more cuts

* Cuts would be popular in 2010 election year (Adds White House reaction)

LONDON, July 16 (Reuters) - Britain could look at further reductions in the number of its nuclear warheads, but only if the United States and Russia make deeper cuts in their arsenals than those already announced, the government said on Thursday.

This month, the United States and Russia pledged to finalize a treaty by year-end to cut deployed nuclear warheads on each side to 1,500-1,675 from levels above 2,200, spurring hopes for a new wave of nuclear weapons cuts across the globe.

Washington will host a nuclear security summit in March where Iran and North Korea are likely to come under pressure from Western nations to sign up to the idea of a nuclear-free world.

"The government continues to judge, as in 2006, that a minimum nuclear deterrent remains an essential element of our national security," Britain's Labour administration said in a policy document aimed at influencing the Washington meeting.

"Once the strategic conditions are established that allow the U.S. and Russia to make substantial reductions beyond those being currently negotiated of their warhead stockpiles, we believe that it is likely to be appropriate for the UK to reconsider the size of its own stockpile."

Britain has about 160 operational warheads, a 75 percent cut from the Cold War era.

"As soon as it becomes useful for our arsenal to be included in broader negotiation, Britain stands ready to participate and to act," Prime Minister Gordon Brown said in a statement.

POSSIBLE SPENDING CUTS

Washington welcomed the policy document but did not specifically address the issue of Britain's nuclear arsenal.

U.S. President Barack Obama agreed it was "essential to act now, in concert with other nations, to ensure the threat of nuclear terrorism is reduced," the White House said in a statement.

"We welcome the resolve stated in the report to address, with our international partners, pressing immediate challenges in the nonproliferation regime such as Iran and North Korea," National Security Council spokesman Mike Hammer said.

There has been speculation that Britain could be considering scaling back or even scrapping its nuclear weapons, especially because it needs to cut spending in some areas to reduce the government's ballooning debt burden.

The government said in 2006 it would replace its Trident submarine-launched missile program, but last week it announced plans for its first defense review in more than a decade, which could lead to cuts in the 36 billion pound ($59 billion) defense budget.

Brown faces an election by mid-2010 in which opinion polls are pointing to a heavy Labour defeat, and a commitment to cut Britain's nuclear deterrent could prove a vote winner.

A poll this week showed most British voters want to scrap nuclear weapons completely rather than replace Trident.

Brown has argued that nations like Iran, which the West suspects has nuclear arms ambitions, should have to prove they have given up such aims as a part of any wider nuclear deal.

In return, existing nuclear powers would provide civilian nuclear energy and work to reduce their own arsenals.

"Over the next few months, we've got to make Iran aware that the entire international community cannot simply accept, without taking the action of sanctions, the development of a nuclear weapon," Brown told lawmakers.

"If Iran is prepared to work on civil nuclear power and abandon its attempt to have nuclear weaponry then the world is ready in my view to make arrangements to work with them."

Britain also wants global leaders to take a harder line against North Korea, which tested a nuclear device in May. (Additional reporting by David Alexander in Washington; Editing by Jon Hemming and John O'Callaghan)

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