JEREMY Corbyn will make the “ultimate decision” on Labour’s approach to Trident if the UK Government forces a vote on the nuclear deterrent before the Opposition has time to complete its defence review, senior party sources have made clear.

It came as Alex Salmond suggested that the SNP at Westminster could form an anti-Trident alliance with those Labour figures, like Mr Corbyn, who opposed retaining and maintaining the deterrent.

During Prime Minister’s Questions, David Cameron ridiculed the Labour leader over Trident, telling MPs that his suggestion that Britain’s nuclear submarine fleet could be retained but without their nuclear weapons was “absurd”.

In answer to a suggestion that Labour’s policy had been inspired by the Beatles song Yellow Submarine, the PM said: “There is a comic element to sending submarines to sea without missiles but this is, in fact, an absolutely serious issue because the deterrent has been, on a cross-party basis, an absolutely key part of our defence and making sure that we have the ultimate insurance policy, which we on this side of the House support and which we should vote on.”

Later, a senior aide to Mr Corbyn was pressed on what would happen if the UK Government staged a vote on the so-called maingate decision on Trident before Labour had finished its review; Emily Thornberry, the shadow defence secretary, is due to publish an interim defence review in June but the final recommendation is due to go to the Labour conference in September for a decision.

He replied: “Ultimately, it’s a decision for the leader in consultation with the shadow cabinet.”

Given that a number of Labour frontbenchers have signalled they could resign if the party policy was changed from the current policy of maintaining Trident to one of scrapping it, then this throws up the possibility that, as with the recent vote on RAF airstrikes in Syria, Mr Corbyn might be forced to allow a free vote.

This idea was not disabused by the aide, who made clear: “All differences will be respected. There’s no reason for anyone to resign over anything.”

He stressed that any decision on the deterrent would not be a simple choice between the existing fleet of submarines armed with warheads and complete disarmament, suggesting that other options - including land-based deterrents and Cruise missiles - would be considered by Ms Thornberry's review.

The Labour source also suggested any Commons vote staged over the coming months would have only "symbolic" importance as the Trident renewal project would continue to be debated in the years to come.

Whitehall sources have suggested that the Conservative Government’s decision on when to hold the key Commons vote was still “up in the air”.

Mr Cameron may hold the vote after the Labour conference when the party has split with possible frontbench resignations, if Mr Corbyn gets his way and the party reverses its policy to oppose renewing Trident. Such a move would mean the UK party falling into line with the Scottish party’s view; albeit one, which is opposed by its own leader, Kezia Dugdale.

The other is to hold the Commons vote before the Holyrood elections.

Meantime, during his phone-in show on LBC, Mr Salmond said: "I certainly hope the Labour leadership is shifting position on Trident although I don’t agree with the Yellow Submarine argument…A straight vote against Trident would be good.” He said he foresaw an alliance between Labour MPs' backing non-renewal of Trident and the SNP.