Jeremy Corbyn has suggested Trident submarines could be sent to sea without nuclear warheads in an apparent bid to convince unions a U-turn on Labour's defence policy need not cost thousand of Scottish defence jobs.

But a furious row erupted as critics inside and outside his party poured scorn on the proposal, with one likening it to an Army with no ammunition.

The ‘no nuke’ idea is being considered by his party's defence review, Mr Corbyn announced.

The proposal was defended by the shadow defence secretary Emily Thornberry, who described it as the "Japanese option", keeping the capacity to build nuclear weapons without actually possessing them.

Pro-Trident Labour MPs greeted the suggestion with derision, while the Conservatives said it showed that Labour was a "threat to national security".

The SNP claimed that Mr Corbyn's personal opposition to nuclear weapons had gone from " red hot to lukewarm".

Former Labour minister Alan Milburn said that the Tories were 'laughing all the way to the bank" at Labour’s shift to the left under Mr Corbyn.

There was also confusion about other parts of Labour’s Trident policy as shadow chancellor John McDonnell refused to say that Labour MPs would get a 'free vote' on the renewal of the ageing weapons system on the Clyde.

Earlier Mr Corbyn has suggested that they would, saying he was not one for "clearing the decks".

The row erupted as Mr Corbyn and Nicola Sturgeon both appeared to woo Unite union boss Len McCluskey, as he appeared at his union’s Scottish conference.

Mr McCluskey also appeared to put Mr Corbyn on notice, saying his leadership of the party should be reviewed if he has not proven himself within three years.

Within hours Mr Corbyn announced that he would overturn laws against so-called 'sympathetic strikes', while Ms Sturgeon said her party’s anti-Trident policy would include a pledge to find workers alternative jobs.

That issue was also Labour’s “first priority” Mr Corbyn said, just a day after Mr McCluskey warned of devastation to communities.

Mr McDonnell appeared to go further, saying that all jobs would be “guaranteed”.

But the Labour leader provoked fury from some of his own MPs when he said that there were a number of options available, adding of the submarines: “They don’t have to have nuclear warheads on them.”

He also restated that he would never give authorisation for nuclear weapons to be used., but added: "I don’t believe David Cameron would use it either."

In a wide ranging interview on the BBC, he also suggested that the UK talk to Islamic State militants and called for "sensible dialogue" with Argentine over the Falkland Islands.

Ms Thornberry defended the idea of maintaining submarines and nuclear capabilities without actually having operational nuclear weapons.

She said: "The way that it works is that the Japanese have got a capability to build a nuclear bomb...[but] you can then put them on to, or you can use them, in various delivery forms. So that’s a possibility that is an option.”

However, she added that there was a “debate to be had about how many jobs are specifically related to the renewal of Trident”.

Despite her comments, Mr McCluskey appeared to welcome the concentration on jobs.

"What Jeremy and Emily Thornberry seem to be saying is that we will have a proper review, a comprehensive review, and we’ll play our part in that," he said.

John Woodcock, the MP for Barrow and Furness, where the replacement submarines are due to be built, described the idea as completely implausible.

"Having a deterrent that has no ability to deter because it has no missiles is like having an army with broken rifles and no ammunition," he said.

"It is deeply frustrating because every day that we spend debating implausible schemes like this is a day we are not able to hold the Conservative Government to account."

Defence Secretary Michael Fallon said Mr Corbyn's comment showed Labour could not be trusted in government.

"It's clearer than ever that Labour are a threat to our national security and our economic security," he said.

"When you see what is going on in parts of Syria and Iraq, (the idea) you could sit down with these people and have a reasonable conversation with them is completely absurd," he said.

The SNP MSP Bill Kidd called on Labour to have a "clear" policy on Trident.

"Jeremy Corbyn has said that he is against Trident but now it appears as though his opposition has gone from red hot to lukewarm with his proposal to keep Trident submarines without the nuclear warheads."