A top defence expert has revealed a weakness in Britain’s nuclear deterrent amid increasing concerns over American commitment to European security. Four Royal Navy submarines are capable of carrying the UK’s nuclear deterrent - Trident missiles - but only one is at sea at one time.

Defence analyst Professor Michael Clarke told BFBS Forces News: "The fact that we've only got one boat on patrol means that if we used our weapons tactically... we could actually fire an unboosted primary of one or two kilotons or four or five kilotons, which would be pretty small by nuclear standards. But using them in a tactical way would reveal where the boat is."

The UK and France are Europe’s sole nuclear powers. The US provides a nuclear umbrella for the continent but with continued worries over Washington’s willingness to support and defend its allies, questions have been raised over whether the UK and France could step up.

In addition to nuclear-armed submarines, France’s fourth-generation Rafale fighter jets can also launch nuclear weapons.

Prof Clarke told BFBS Forces News: "What keeps NATO deterrent alive and credible, it is said, is the fact that any aggressor has got to take account of three decision centres rather than just one.

"So even if the United States were not involved, I mean that's a big if, of course, these days, but if they were not involved, and it's always been assumed they would be, there is still 'what would London and Paris do?'."

It comes as a former US ambassador to NATO warns that allied faith in Washington has “shattered” and that it is “very possible” the Trump administration could leave the alliance.

Ivo Daalder told the Express: “But the point is not will the US withdraw, the point is will allies still believe that the US is a credible partner and that belief is already shattered.”

The Navy has an ageing fleet of four Vanguard-class submarines that can carry the Trident nuclear deterrent.

Last month, one of the boats, HMS Vanguard, returned home after a record-breaking 204 days at sea.

Deployments of the almost 150-metre-long submarines have been increasing in recent years amid reported issues with maintenance.

Since April 1969, the Royal Navy has maintained continuous at-sea deterrence, with at least one nuclear-armed ballistic missile submarine patrolling the seas.