Shadow ministers will not have to resign if they oppose Labour leader Jeremy Corbyn's position on the renewal of Britain's Trident nuclear deterrent, a senior party source has said.
The comment is the strongest indication yet that Mr Corbyn may offer his MPs a free vote in any Commons vote on Trident which comes before the completion of the party's ongoing review of its defence policy.
There is widespread expectation in Westminster that ministers will force a vote on Trident in the coming weeks, before the recommendations of the review are announced by shadow defence secretary Emily Thornberry in June or ratified by annual conference in September.
Mr Corbyn was taunted at Prime Minister's Questions in the Commons over his suggestion in a weekend TV interview that the UK could retain its nuclear missile submarines but send them to sea without their warheads, which Tory backbencher Karl McCartney described a "Yellow Submarine" policy.
Taking up the Beatles theme, Prime Minister David Cameron responded that Mr Corbyn might prefer "Back In The USSR", adding: "The deterrent has been, on a cross-party basis, an absolutely key part of our defence and making sure we have got the ultimate insurance policy which we support on this side and we should vote on in this House."
Senior Downing Street aides later declined to comment on speculation that a vote on Trident's "maingate" - approval for the project to move into the manufacturing stage - might come as early as next week.
Asked whether Labour's existing policy of support for Trident would be applied in any such vote, the senior party source described it as "a policy in review, which puts it in special circumstances" and said it would be for the leader himself to decide on what approach Labour would take.
"The policy is in review and if there is a vote in Parliament, depending on how it is framed, there will then be a decision about how to deal with that," said the source.
"But Jeremy Corbyn has made clear repeatedly that it is going to be an open process and all differences will be respected in the shadow cabinet and the whole party and the whole Parliamentary Labour Party and that there is no reason for anyone to resign or anything like that.
"Judging how to approach such a vote would depend on its terms."
Whipping arrangements in a vote on Trident would be a matter for the leader, but would be agreed in consultation with the shadow cabinet in a process which has already begun, said the source.
Asked whether Labour MPs would be offered a free vote, he said: "Nothing has been said about that, but Jeremy Corbyn has emphasised that all positions in an issue which people obviously have different views about in the Labour Party - as they do in other parties and as they do in the military - will be fully respected when a vote takes place.
"Those views will be respected, there will be no reason for anyone to resign, but the actual particular response will depend on what the proposal that's brought forward by the Government is."
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 claimed the Ministry of Defence and Treasury were "at sixes and sevens" over the procurement process and suggested that 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.