Britain must be prepared to fight future wars without the United States as its principal ally, Defence Secretary Ben Wallace has warned.

Mr Wallace said the increasing withdrawal of America from international leadership under Donald Trump meant Britain needed to re-think the assumptions underpinning its defence planning for the past decade.

His warning came as Prime Minister Boris Johnson prepares to lead what is being billed as the "deepest review" of Britain's security, defence and foreign policy since the Cold War.

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In an interview with The Sunday Times, Mr Wallace said the prospect of the US stepping back from its international role "keeps me awake at night".

"I worry if the United States withdraws from its leadership around the world. That would be bad for the world and bad for us. We plan for the worst and hope for the best," he said.

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Mr Wallace said Britain should use the defence review to acquire new capabilities, making it less dependent on the US in future conflicts.

"Over the last year we've had the US pull out from Syria, the statement by Donald Trump on Iraq where he said NATO should take over and do more in the Middle East," he said.

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"The assumptions of 2010 that we were always going to be part of a US coalition is really just not where we are going to be.

"We are very dependent on American air cover and American intelligence, surveillance and reconnaissance assets. We need to diversify our assets."