The Trident nuclear deterrent on the Clyde must be kept to prevent any future threat posed by Russia, a cross-party commission of MPs has concluded.

Last year Deputy Prime Minister Nick Clegg questioned the need to spend billions updating a weapons system designed to "flatten Moscow". But since then the Russian government has attracted internal controversy with its incursion into Ukraine.

The warning is just one conclusion from a report by the Trident Commission.

It also suggests the weapons system could be scaled back when it is up for replacement in a few years.

The Commission, co-chaired by the Conservative Sir Malcolm Rifkind, former Labour minister Lord Browne of Ladyton and the Liberal Democrat Sir Menzies Campbell, rejects any alternative to Trident.

Its conclusions were condemned by Deputy First Minister Nicola Sturgeon, who said September's independence referendum was a chance to remove nuclear weapons from Scotland.

"The Scottish Government position is Trident should be removed from an independent Scotland by 2020... (and) a prohibition on nuclear weapons being based in Scotland, ensuring they would never return," she said.

She said Scottish taxpayers should not be hit with the multi-billion cost of replacing Trident.

"That is money that could and will be far better spent on other priorities," she said.

A decision on a Trident replacement is due in 2016.

The Conservatives and Labour back a full-scale like-for-like replacement, which could cost about £20 billion. They argue it is necessary to keep the nuclear deterrent continuously at sea.

However, the LibDems have argued for that requirement to be scrapped and a smaller fleet of three or even two boats.

The Commission's report raises the possibility of scaling back the size of the fleet and suggests the UK could even negotiate shared responsibility with America and France,

It adds: "We believe the crucial consideration for the British Government in deciding upon the renewal of its nuclear deterrent is national security.

"If there is more than a negligible chance that the possession of nuclear weapons might play a decisive future role in the defence of the United Kingdom and its allies, in preventing nuclear blackmail, or in affecting the wider security context within which the UK sits, then they should be retained."

The report also called on the Ministry of Defence to consider any possible steps it could take to work towards multilateral disarmament.

Vernon Coaker, the shadow defence secretary, said his party was committed to ensuring the retention of a "minimum credible independent nuclear deterrent, which we believe is best delivered through a continuous at sea deterrent."

But he added that between now and 2016 Labour would continue to examine ways to deliver a Trident replacement in the most cost efficient way possible.

Kate Hudson, general secret­ary of the Campaign for Nuclear Disarmament, said: "The Trident Commission's conclusion that the UK should retain and deploy a nuclear arsenal demonstrates Westminster's pro-Trident mindset.

"It repeats the failings of the Liberal Democrats' Trident Alternatives Review, the Coalition's Strategic Defence and Security Review and the previous Labour Government's White Paper, all of which failed to articulate a convincing case for retaining a nuclear weapons capability.

"In contrast to proposals in the Trident Commission, only imaginative new steps can prevent us from further isolating ourselves from the rest of the world and potentially jeopardising the future of the Non-Proliferation Treaty.

"Cancelling the programme to replace Trident nuclear weapons is a pragmatic and realistic alternative."

The UK has four Trident submarines, based at Faslane. At any one time one is armed and at sea, with one undergoing maintenance and two in port or on training.