The Government must increase spending on the armed forces to 3 per cent of GDP if Britain is to maintain its defence relationship with the United States, MPs have warned.
The Commons Defence Committee said without further investment, UK forces would struggle to maintain the UK-US military relationship.
The warning comes ahead of next month's NATO summit in Brussels where US President Donald Trump is expected to reiterate demands for European allies to increase their NATO defence contributions.
The committee repeated its call for the Government to raise defence spending from the NATO minimum of 2% of GDP to 3% - around £60 billion a year.
"In order to fill the existing black holes in the equipment plan and elsewhere, UK defence spending would have to rise to 2.5% of GDP," it said.
"However, significantly to improve the capacity—as well as the capability—of the UK Armed Forces, defence spending would need to rise closer to 3% of GDP.
"Diminished capacity reduces the UK's usefulness to the US and our influence within NATO. The Government must not allow this to happen."
A defence spending of 3% of GDP was last achieved by Britain in 1995-96.
The UK 'Might Cease To Be A World Military Power'
General Lord Houghton, former head of the armed forces, has also warned ministers they must increase spending on defence if Britain is to remain a world military power.
He told the BBC Radio 4 Today programme that the UK was at a "strategic crossroads" and the Government had to decide what sort of country it wants to be in a post-Brexit world:
"We stand at a strategic crossroads. We have got to come off the fence one way or another.
"It might be, and it is a wholly worthy opinion, that the United Kingdom should cease to be a world military power.
"We stand in a period where, to be honest, our sense of national identity and credibility is wobbling.
"We need to have a strategic decision that flows from government about what sort of a place and a country the UK wants to be going forward.
"To me, that is best served as a global Britain, as a strong military power, as a good alliance member, as a country that will help secure the stability of the world."
His intervention comes amid reports that Theresa May and Chancellor Philip Hammond have clashed with Defence Secretary Gavin Williamson over the level of military capability the UK needs to maintain.
After the Prime Minister's promise of £20 billion-a-year extra for the NHS, allies of Mr Williamson on the Tory backbenches are reportedly threatening a revolt if there is no additional funding for defence in the Budget in the autumn.
The US-UK Defence Gap
The committee said that US Defense Secretary James Mattis had estimated that the UK benefited to the tune of £3 billion-a-year from its defence relationship with the US.
"This implies that both the UK armed forces and HM Treasury benefit from our close relationship with the US."
"However, that will continue to be true only while the UK military retains both the capacity and capability to maintain interoperability with the US military and to relieve US burdens.
"For this to be the case the UK armed forces must be funded appropriately."
Earlier this month, US President Donald Trump highlighted that European nations spend considerably less on defence compared to America’s 4% of GDP.
Mr Trump called the current arrangement a rip-off and warned that "change is coming".
Rt Hon John Spellar MP, Defence Committee senior Labour member and former Minister for the Armed Forces, said:
“We have heard that there are perceptions in the US that the UK’s defence capabilities have slipped and that concerns have been raised about the UK’s ability to operate independently.
“We need to challenge this perception and the Modernising Defence Programme is an excellent opportunity to do so.”
Rt Hon Dr Julian Lewis, Defence Committee chairman, said:
“The Government has consistently talked about increasing the UK’s commitment to NATO after our departure from the European Union.
“An increased commitment, in the face of new and intensified threats, means that further investment is essential.
“Where percentage of GDP for Defence is concerned, our mantra must be: ‘We need 3, to keep us free’. Anything less is simply rhetoric which endangers us and our allies.”
A spokesman for the Ministry of Defence said
"The UK maintains the biggest defence budget in Europe we have been clear we will continue to exceed NATO's 2% spending target.
"The Defence Secretary launched the Modernising Defence Programme to strengthen our Armed Forces in the face of intensifying threats."