A Labour former defence minister has described leader Jeremy Corbyn's suggestion that nuclear submarines could be retained without having to carry warheads at all times as "ill-informed".

Kevan Jones, who quit the Labour frontbench after pro-Trident Maria Eagle was removed from her shadow defence secretary role, warned that you cannot turn the nuclear deterrent "on and off like a tap".

He spoke as Ms Eagle's replacement and Trident sceptic Emily Thornberry made her Commons debut at defence questions.

Mr Corbyn's plan appeared designed to win over trade unions who fear that scrapping Trident - as the Labour leader wishes - would destroy tens of thousands of jobs in the defence industry.

Ms Thornberry, who is leading the party's defence review, has confirmed she is looking at the "Japanese option" - retaining the capacity to build nuclear weapons without actually possessing them.

But defence minister Philip Dunne backed Mr Jones's insistence that jobs cannot be "switched on and off".

Mr Jones told MPs: "Would you agree that it's not just about the number of jobs involved in the successor programme but the high-skilled nature of those jobs?

"Despite ill-informed comments from my own party at the weekend with regards to these jobs, would you also agree with me that simply you can't turn them on and off like a tap when you need them?"

Mr Dunne replied: "I'd like to add my tribute of your stalwart work in your position both on these when you were a defence minister and on those benches when you were a shadow.

"And it's a sorry state to see you sitting right at the back of the back benches today.

"You are of course quite right to point out that this is a long-term endeavour, to design and build a nuclear-enabled submarine takes decades and this is a 35-year project from initial conception to commissioning.

"And that those skills not only take a long time to develop, they can't be switched on and off, and they are the very forefront of engineering capability in this country.

"Building a nuclear submarine is more difficult than sending a man to the moon."

Mr Dunne described Mr Corbyn and Ms Thornberry's position as an "extraordinary contortion" from the "champagne socialist salons of Islington" - a reference to the pair's constituencies in north London.

He said the Opposition leader's idea for the future of Britain's nuclear submarines showed a "breathtaking lack of understanding" of national security.

In reply to Tory Michael Fabricant (Lichfield), Mr Dunne said: "We have seen, I think you were referring to comments over the last 24 hours, the most extraordinary contortion emerging from the champagne socialist salons of Islington.

"The idea that spending tens of billions of pounds to build but not arm a strategic deterrent betrays the new kind of politics from the Labour leadership - a lurch back to the discredited unilateralism of the 1970s, a breathtaking lack of understanding of how to keep this country safe and the consequent threats both to national security and to tens of thousands of jobs across the UK."

Earlier, Defence Secretary Michael Fallon formally welcomed Ms Thornberry to her post but criticised the stance of Labour's new defence team.

He said: "I hope, Mr Speaker, you will allow me to formally welcome the new shadow secretary and her team and regret the removal of their mainstream moderate predecessors Ms Eagle and Mr Jones."

But Ms Thornberry hit back, saying: "I'd like to begin by thanking you for your generous welcome to the job...

"You have the honour of having perhaps the best job in Westminster, mine is the second best, hopefully we will change roles fairly soon."

Mr Fallon claimed Britain would never vote for Labour, referring to Mr Corbyn's suggestion that a "back channel" communication line could be opened up with Islamic State, also known as Daesh, and that a "sensible dialogue" be opened with Argentina over the Falkland Islands.

Turning to Ms Thornberry, he said: "Let me thank you for your initial remarks and note your ambition to move from that side of the House to this side of the House - presumably shared by the two previous shadow defence secretaries that I've so far come across.

"Let me just say to you very gently that a defence policy of nuclear submarines with no nuclear weapons, that thinks that Daesh have strong points and wants to end the Falkland Islanders' right to self-determination may be Labour's defence policy but it will never be Britain's defence policy."

Conservative former cabinet minister David Jones mocked the idea of submarines without nuclear weapons, questioning how such an approach would deter North Korean aggression.

The Clwyd West MP told Mr Fallon: "North Korea recently announced it had tested a hydrogen bomb and only yesterday boasted it had the capacity to obliterate the United States."

To laughter from MPs, Mr Jones added: "To what extent do you think that North Korea would be deterred in its nuclear ambition by the knowledge that somewhere below the surface of the East China Sea an unarmed submarine was lurking?"

Mr Fallon condemned North Korea's nuclear test, adding: "This Government, let me assure you, will not gamble with the long-term security of our citizens.

"We remain committed to maintaining an independent nuclear deterrent.

"The only thing that a nuclear submarine without nuclear weapons is likely to deter is anybody who cares about our security from ever voting Labour again."

Mr Fallon said talk of the UK adopting an approach similar to Japan is "entirely farcical".

Labour's John Woodcock, who represents Barrow and Furness, where the submarines for Trident are built, asked Mr Fallon: "If the UK were to go down the route of decommissioning its warheads and then in the so-called Japanese style were to decide that it needed to recommission them at a certain future point, is it the Government's assessment that it could do so and remain compatible with the non-proliferation treaty?"

Mr Fallon replied: "Japan does not have nuclear-powered submarines and Japan does not have nuclear weapons, so talk of some Japanese option is entirely farcical."

He added: "We have no intention of decommissioning."