Donald Trump’s military chief has stressed the importance of Britain’s defence capabilities remaining “credible” during a meeting with Defence Secretary Gavin Williamson.

US Secretary of Defence Jim Mattis said the so-called “special relationship” should be left “stronger than we inherited it” during a meeting at the Pentagon.

The Government is facing growing warnings over Britain’s ability to defend itself as it faces evolving threats from conventional enemies and escalating risks from cyber attacks.

Earlier this week a watchdog warned the Ministry of Defence was facing a £20 billion black hole in the armed forces equipment budget that could jeopardise operations unless urgent action is taken.

The National Audit Office (NAO) report came after the head of the Army warned Britain needs to keep up with its enemies, as it is not immune to a “hybrid” attack that could use conventional and cyber warfare methods.

Meanwhile, the head of Britain’s cyber defence centre has warned a major cyber attack on the UK is a matter of “when, not if”.

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As the defence secretaries met in Virginia on Thursday, Mr Mattis said he and Mr Williamson would be discussing “pragmatic ways to bolster our defence cooperation as we carry out our responsibility to strengthen the special relationship, turning it over to the next generation stronger than we inherited it”.

Chief Pentagon spokeswoman Dana White said after the meeting: “Secretary Mattis emphasised the value of the US-UK special relationship and reaffirmed the importance of credible defence capabilities.

“He also welcomed the release of Britain’s forthcoming report of defence programme modernisation.”

There has been widespread speculation about possible cuts to UK armed forces personnel and equipment in a coming review to relieve major pressure on the defence budget.

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There have been calls from some MPs to increase defence spending to 3% of GDP, and Mr Williamson has said “hard work” is taking place across Government to give the “right resources” to the armed forces.

However reports in the media suggest there are plans to cut the armed forces’ strength by more than 14,000, as well as the joining of elite units of paratroopers and Royal Marines to save cash.

On Wednesday the NAO said that unless urgent action was taken to adjust the £180 billion, 10-year plan projects would face delay or cancellation, jeopardising the forces’ operational capability.

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Mr Williamson said after the meeting that the US and UK shared the “closest” defence relationship and pledged to keep Britain safe.

“Our two countries face the same threats in an increasingly dangerous world,” he said.

“We stand shoulder to shoulder on the battlefield, we share vital intelligence and technology, and our century-long relationship is the closest of any two allies across the globe.

“Together, we will always keep the streets of the UK and the US safe, protect our citizens, and defend our way of life.”