Britain will remain a “tier one” military power with the ability to deploy forces around the world, Defence Secretary Gavin Williamson has said.

Speaking on a visit to Washington where he was holding talks with US defence secretary general James Mattis, Mr Williamson said the UK would continue to be “reliable partners for the long term”.

His comments came after Theresa May caused consternation earlier this year when she refused to state explicitly that the UK would remain a “tier one” power amid reports of clashes with Mr Williamson over future defence spending levels.

Addressing the Atlantic Council think tank, Mr Williamson said: “Britain is a major global actor. We have always been a tier one military power and we always will be a tier one military power.”

He pointed to the UK’s independent nuclear deterrent, “world class” special forces and cyber capabilities, and “exceptional” conventional forces “able to deploy indpendently around the globe”.

Mr Williamson also used his address to empahsise the continuing importance of Nato to the US after Donald Trump berated allies for failing to contribute more to the costs of their collective defence at last month’s alliance summit in Brussels.

The Defence Secretary pointed out that the only time Nato’s Article 5 – which commits members to come to the defence of an ally which is attacked – had been invoked was after the 9/11 terror attacks on the Twin Towers in 2001.

He said years of pressure by the US and the UK to persuade other allies that they needed to spend more on defence were now paying off, with eight member states set to meet the target of spending 2% of their GDP on defence.

“Alongside the US, the UK has been pressing for the alliance to do more, to pay its way. We are now seeing the results. Last year saw Nato’s biggest spending increase in 25 years,” he said.

“Since making the investment pledge at the 2014 Wales summit, allies have spent 87 billion dollars more on defence. In just two years time that number will increase to at least 150 billion dollars.

“Increasingly we are seeing more partners pull their weight realising that they have got to spend more because of the increasing threat that the world faces.”