01/02/2016 13:01 GMT | Updated 01/02/2016 13:59 GMT

Jeremy Corbyn's 'Submarines With No Nukes' Plan Deemed Too Difficult To Deliver, Emily Thornberry's Defence Review To Signal

Embargoed to 0001 Sunday January 3File photo dated 30/01/02 of the Royal Navy's 16,000 ton Trident-class nuclear submarine Vanguard. Costs for the "immoral and obscene" replacement for the Trident nuclear missile submarines have already reached almost £5 billion before Parliament has voted to give the project the final go-ahead, the SNP said.

Labour’s review of defence policy has concluded that Jeremy Corbyn’s option of ‘submarines without nuclear weapons’ would be very difficult to achieve, HuffPost UK has been told.

Shadow Defence Secretary Emily Thornberry, who is set to brief the Shadow Cabinet on her analysis tomorrow, is understood to have swiftly realised that leaving nuclear warheads off the new generation of Successor submarines would be impractical.

Last month on the Andrew Marr Show, Mr Corbyn floated the idea of a ‘third way’ on planned £1bn renewal of the UK’s Trident nuclear deterrent system.

The Labour leader had stressed that Trident didn’t need a ‘binary’ decision on the submarines upgrade, adding “they don’t have to have nuclear warheads on them”.

Mr Thornberry later referred to a ‘Japanese option’ of ‘various delivery forms’ of having submarines without armed warheads.

But after weeks of deliberation on the subject, Labour sources say that the Shadow Defence Secretary’s review has come up with some early deliberations, including the realisation that most of the £31bn budget would be spent on building the new Successor submarines – and that simply leaving off the nuclear warheads from the missiles would not save much money.

Under pressure from unions like Unite and the GMB, Mr Corbyn has stressed he wants to retain the skilled manufacturing jobs and supply firm jobs that Trident renewal would deliver.

Renewal would mean replacing the current Vanguard class of subs with bigger and more expensive Successors.

One Labour source told HuffPost: “Emily has worked out that this really is a binary choice after all. You either renew it, to get the jobs, or you don’t and you save the billions you want.”

Other sources stressed that Ms Thornberry had not ruled out alternatives, but had realised the complexity of the defence supply system needed for the project.

When Mr Corbyn first floated the idea of nuclear subs without nuclear weapons, he met with a volley of ridicule from Defence Secretary Michael Fallon and from David Cameron.

It is understood, however, that Ms Thornberry is still leaving open other options, but it is proving difficult to make them financially or politically coherent.

HuffPost UK has been told that she is determined to pursue the other alternatives, which will give Mr Corbyn and other nuclear sceptics in the party something to work with in terms of a 'third way'.

The final report of the defence review is due to be delivered by June, in time for the crucial Unite policy conference in July and party conference this autumn.

John Woodcock, the Labour MP whose Barrow constituency builds the subs, said at the time: “Having a deterrent that has no capacity to deter is like having an army with broken rifles and no ammunition.”

But Caroline Lucas, the Green party MP who is now chair of the Campaign for Nuclear Disarmament (CND), said the Corbyn ‘third way’ sounded like a “really uncomfortable compromise” and an “extraordinarily expensive job creation scheme”.

A Commons vote on Trident renewal by the end of March is being pushed by the Ministry of Defence within Whitehall, although some in Government want a delay until Labour’s party conference votes on the policy.

Ms Thornberry, who is on record as being a unilateralist, was handed the Shadow Defence Secretary post last month, ousting Maria Eagle, who was a firm supporter of Trident renewal.

At Labour's Parliamentary Labour Party (PLP) on Monday night, deputy leader Tom Watson warned MPs that the overwhelming message from Scotland and around the country was that the party wanted 'infighting' at Westminster to stop.

Both Labour MSPs and Labour councillors had told him that voters on the 'doorstep' had fed back that the image of disunity was huring the party.

Mr Watson told MPs that for the next 90 or so days to the May elections, party unity was more important than ever.