Philip Hammond, the defence secretary, will reignite the argument over Britain's independent nuclear deterrent on Monday when he announces a further multimillion-pound contract for a new generation of nuclear missile submarines, making it clear he plans to press ahead with a Trident replacement.

The Ministry of Defence said the £350m contract would sustain 1,200 UK jobs, adding that the investment made "clear the government's firm commitment to maintaining continuous at-sea deterrence for future decades".

Hammond, who will visit the Trident submarine base at Faslane on the Clyde on Monday, said: "Our continuous submarine-based nuclear deterrent is the ultimate safeguard of our national security and the government is committed to maintaining it, both now and in the future.

"This latest expenditure for the next generation of nuclear-armed submarines is an investment in UK security and the British economy, sustaining high-quality jobs and vital skills."

He added: "We are confident that the Scottish people will choose to remain part of the United Kingdom."

The remarks are likely to be viewed as a sign that Hammond intends to ignore a government-commissioned study into a Trident replacement if it fails to support a like-for-like replacement. The Cabinet Office study is due early next year, and Liberal Democrats had been hoping that senior military officials in the MoD might be persuaded to back a cheaper replacement than like-for-like renewal if a cogent case was assembled.

There were hopes that senior figures in the army might also oppose such an expensive commitment. But in a blow to those hoping that the MoD could be persuaded from within, it was decided by the deputy prime minister, Nick Clegg, to sack the armed forces minister Nick Harvey in the autumn reshuffle, and leave the MoD without a Lib Dem minister.

Responsibility for the study has now been handed to the Treasury chief secretary, Danny Alexander.

Clegg has insisted the reshuffle did not indicate any lessening of his commitment to find a cheaper replacement for Trident. He told his party conference: "I am more determined than ever to find the right alternative to such a monumentally expensive replacement for a cold war deterrent."

A Lib Dem official said on Sunday that discussion about the replacement for Trident was still needed. "No final decision on the replacement of Trident will be made until 2016.

"Instead of blind faith in a cold war relic costing billions of pounds, we should be having a debate on how best to maintain our nuclear deterrent in the modern world."

The official added: "The review being led by the Liberal Democrats in government will inform that debate when it reports next year."

Other senior Lib Dems were concerned that the defence secretary was using his status to insist on a full replacement.

Menzies Campbell, the former leader, said: "There is no doubt about the terms of the agreement between both parties in the coalition that the 'main gate decision' on a replacement for the nuclear deterrent is not to be made until 2016. Danny Alexander and, before that, Nick Harvey have been tasked to look into alternatives to a like-for-like submarine. Liberal Democrats, including myself, would expect that agreement to be maintained."

Hammond, who succeeded Liam Fox as defence secretary in October last year, will frame the announcement of extra spending on a possible Trident replacement as a boost for those opposing an independent Scotland, as the successor project sustains hundreds of jobs on the Clyde.

The £350m contract is part of the £3bn awarded last year to BAE Systems to pursue work on a new Trident fleet.

To drive home the point, Hammond will announce government plans to make Faslane the base for the UK's fleet of nuclear-powered but conventionally armed Astute and Trafalgar class attack submarines, as well as for Trident. The move will create a further 1,500 jobs in addition to the existing 6,500 at Faslane.

Admiral Sir Mark Stanhope, the head of the navy, made clear he strongly supported plans to build a new fleet of Trident submarines, officially estimated to cost up to £25bn, despite the lack of a joint agreement by the coalition government to press ahead.

"One of the core roles of the Royal Navy, the continuous at-sea deterrent, remains an enduring strategic capability, underpinning our nation's commitment to the preservation of peace in our uncertain world," Stanhope said.

BAE Systems, which has shed jobs in the fast jet aircraft market, will say on Monday that it is now looking for mechanical, electrical power, propulsion, quality and safety engineers, and naval architects to fulfil the latest contract.

The Scottish Campaign for Nuclear Disarmament (SCND), which obtained new figures from a Freedom of Information Act request to the MoD, said Labour and the Conservatives were trying to scare the public by exaggerating the economic implications of nuclear disarmament. Just 520 civilian jobs at Faslane and nearby Coulport were directly dependent on Trident, the SCND said.

Stephen Boyd, assistant secretary at the Scottish Trades Union Congress, which commissioned an expert study along with SCND into the economic consequences of cancelling Trident, told the Sunday Herald that suggestions as many as 11,000 jobs would be lost in Scotland if Trident were not replaced were inaccurate.

Britain's nuclear weapons system is made up of four Royal Navy Vanguard submarines, based at the Faslane naval base on the Clyde, which can deploy Trident ballistic missiles carrying nuclear warheads.

SNP MSP Bill Kidd, a vice-president of the international organisation Parliamentarians for Nuclear Non-proliferation and Disarmament, said: "For the UK government to boast about spending hundreds of millions of pounds on weapons of mass destruction – while at the same time implementing brutal welfare cuts and slashing investment in the economy – is obscene.

"More than that, Philip Hammond's weak attack on the Scottish people's choice in the independence referendum continues to use fantasy figures relating to the number of jobs associated with Trident at Faslane."