Image copyright PA Image caption HMS Vanguard is one of the four UK submarines carrying Trident missiles

Britain must keep its Trident nuclear weapons system if it wants to play a significant role in the world, the US defence secretary has said.

Ash Carter told the BBC it was an "important part of the deterrent structure of Nato" and allowed the UK to punch above its weight.

MPs are expected to vote this year on whether to back government plans to renew the UK's four Trident submarines.

Labour is currently reviewing its support for the weapons.

Renewing the Trident fleet, which is due to become obsolete by the end of the next decade, is estimated by the government to cost £31bn, although opponents claim the final bill will be far higher.

Asked whether the UK should be investing in a new fleet of submarines amid stretched defence resources, Mr Carter replied unequivocally that it should.

He said Trident aided the UK's "special relationship" with the US and helped it "continue to play that outsized role on the global stage that it does because of its moral standing and its historical standing".

"It's important that the military power matches that standing and so we're very supportive of it," he said.

Underwater drones claim

Mr Carter said the UK and US each had independent authority to use Trident but were "dependent upon one another industrially".

Media playback is unsupported on your device Media caption Ash Carter: Trident "helps the UK continue to play that outsized role on the global stage"

"We depend upon the United Kingdom, the United Kingdom depends on us, that's part of the special relationship," he told the BBC. "We build joint strike fighters together, we build Trident missiles together."

Labour leader Jeremy Corbyn is a longstanding opponent of nuclear weapons and has commissioned a review by shadow defence secretary Emily Thornberry.

But some of the party's MPs and peers have threatened to resign if it reverses its decades-long support for the missiles. Shadow home secretary Andy Burnham, who backs renewal, recently indicated it may be "impossible" for Labour to reach an agreed position on Trident.

The US defence secretary was asked about Ms Thornberry's suggestion that underwater drones could be able to detect submarines in future, making them vulnerable to attack and obsolete.

Mr Carter, who this week attended a meeting with Nato defence ministers in Brussels, said the ability of submarines to "operate stealthily" was proven.

"We are, in fact, making large investments in undersea forces because of their survive-ability, as well as their power," he said.

What is Trident?

Image copyright MOD

Since 1969, according to government documents, a British submarine carrying nuclear weapons has always been on patrol, gliding silently beneath the waves, somewhere in the world's oceans.

The logic is to deter a nuclear attack on the UK because, even if the nation's conventional defence capabilities were destroyed, the silent submarine would still be able to launch a catastrophic retaliatory strike on the aggressor, a concept known as mutually assured destruction.

Four UK submarines carry up to eight Trident missiles; each can be fitted with a number of warheads.

A guide to Trident and the debate about its replacement