Gen McMaster, who served as national security adviser between 2017 and 2018, said: “I think it’s quite urgent for the United Kingdom, the United States, for all nations to invest more in defence.

“What we’ve seen is we have a huge gap in our defence, industrial base, and in the capacity of all of our services, as well as suffering from a bow wave of deferred modernisation.

“So I go back to what George Marshall said about the lead into the Second World War. He said: ‘When you have the time, you don’t have the money. And when you have the money, you don’t have the time.’

So it’s important, I think, to spend now on defence, recognising that it’s a heck of a lot cheaper to prevent a war than to have to fight.”

When asked if he believed the UK should raise defence spending to three per cent, General McMaster said: “How about four per cent? That would be a historic low relative to Cold War levels.”