LONDON (Reuters) - Lawmakers will vote on Monday on whether to renew Britain's nuclear deterrent, a key decision for Western security policy, which partly hinges on Britain as one of western Europe's only two nuclear powers.

Parliament, where the Conservatives have a majority of 16, is likely to approve renewing the Scottish-based nuclear-armed Trident submarines over the opposition of the Scottish National Party and many lawmakers from the Labour Party.

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

The newly appointed prime minister, Theresa May, is expected to tell lawmakers on Monday that the nuclear threat is growing and urge them to put ideology aside and back renewal.

“It is impossible to say for certain that no extreme threats will emerge in the next 30 or 40 years to threaten our security and way of life," May will say, according to pre-released quotes issued by her office.

"We cannot abandon our ultimate safeguard out of misplaced idealism."

Labour leader Jeremy Corbyn, who has been challenged by two candidates seeking to take the helm of the center-left party, is opposed to nuclear weapons as a long-standing anti-war campaigner.

Some military officials also oppose the outlay on Trident, saying the money would be better spent on maintaining the army and on more conventional technology, both of which have recently suffered cutbacks.

Defence firms BAE Systems <BAES.L>, Rolls-Royce [RROYC.UL] and Babcock <BAB.L> would all be likely to benefit from a renewal, with the new submarines expected to enter service in 2028 at the earliest.

A lawmaker's and Reuters' calculations, based on official figures, indicate that the overall cost of replacing and maintaining Britain's nuclear deterrent will reach 167 billion pounds ($220 billion) over 32 years.

The Defence Ministry has not published its own cost estimate for the lifetime of the four submarines, but has said it will take up about 6 percent of the annual defense budget. In March, it put the cost of procurement at 31 billion pounds, plus almost 4 billion already allocated to the design process.

(Reporting by Costas Pitas; Editing by Kevin Liffey)