By Robin Emmott and Wiktor Szary

WARSAW (Reuters) - Britain's outgoing prime minister David Cameron set a July 18 vote for parliament to decide on renewing Britain's nuclear deterrent, in a surprise move seemingly timed to underscore London's commitment to European security at a NATO summit.

Political divisions about whether to replace the Trident submarines, backed in principle by parliament in 2007, have raised questions about Britain's standing as a world power, amplified by Britain's vote to leave the European Union.

Speaking at a NATO summit, Cameron said it was time to put "beyond doubt" the decision on the renewal of the aging fleet of four submarines carrying nuclear weapons.

"Today I can announce that we'll hold a parliamentary vote on July 18 to confirm members of parliaments' support for the renewal of a full fleet of four nuclear submarines capable of providing around-the-clock cover," Cameron told a news conference at the summit.

Cameron, who will hand over to a new prime minister in September, also defended his decision not to leave a new British leader to call the vote, which is expected to pass because of strong support from the governing Conservative party.

The center-left Labour Party had been a supporter of renewal but its new leader, far-left veteran lawmaker Jeremy Corbyn, an anti-war campaigner, is opposed to the plans. The Scottish Nationalist Party wants Britain's Scotland-based Trident submarines scrapped.

"The nuclear deterrent remains essential in my view, not just to Britain's security, but as our allies have acknowledged here today, to the overall security of the NATO alliance," said Cameron, who resigned after last month's EU referendum.

Cameron made the announcement as the North Atlantic Treaty Organisation underlined the importance of its nuclear deterrent in the summit's final statement, toughening the language of a communique two years ago, in an indirect warning to Russia.

NATO allies have been critical of what they say is reckless talk by Moscow about its nuclear prowess. Alliance Secretary-General Jens Stoltenberg is concerned Russia could be lowering the bar for using nuclear weapons.

"NATO has the capabilities and resolve to impose costs on an adversary that would be unacceptable and far outweigh the benefits that an adversary could hope to achieve," the statement said.

"The independent strategic nuclear forces of the United Kingdom and France have a deterrent role of their own and contribute to the overall security of the alliance."

Britain's Conservative government says the nuclear deterrent is vital to keep Britain safe in an increasingly hostile world, but some opposition figures say it is indefensible to spend billions on renewing the program at a time of austerity cuts.

The government has indicated the price tag for replacing the fleet has risen since 2007 but has not given a full cost over its expected 30-year life. Calculations by Reuters and a Conservative lawmaker suggest it could reach 167 billion pounds.

(Reporting by Robin Emmott and Wiktor Szary; editing by Andrew Roche)