The UK risks losing its special relationship with America unless it boosts its defence spending, a senior US official has warned.

James Mattis, the American defence secretary, has written to his British counterpart, Gavin Williamson, warning of the dangers of not investing in military capability.

In the letter, which was delivered two weeks ago, Mr Mattis says Washington is “concerned” that Britain’s military prowess is “at risk of erosion”.

He said that, with the threats to the world increasing, America expects the UK to increase spending on defence to help counter them “beyond what we would expect from allies with only regional interests”.

The general also makes it clear that while Britain may want to “remain the US partner of choice”, France is already boosting its defence spending - and is waiting in the wings to replace us.