Britain has been warned to boost its defence spending to 4 percent as "we are on the cusp of another World War".

US national security adviser General HR McMaster also told LBC's Andrew Marr that the UK should invest in an Iron Dome-style air defence system.

He said: "How about 4 percent? That would be a historic low relative to Cold War levels.

"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 modernization.

"So, I go back to what George Marshall said about the lead into World War II, 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."

The UK will increase defence spending to 2.5 percent of national income by 2030 - a move that has been seen as less than ambitious by some insiders.

Last week, Defence Secratery Grant Shapps said an increase to 2.5 percent of GDP will allow Britain to use its “military muscle” alongside its allies.

In a statement, he told the Commons: “We increasingly need our armed forces, and we increasingly are asking more of them. So yesterday, the Prime Minister committed to hit the 2.5 percent of GDP for defence by 20230. It means we’ll invest an additional £75billion into defence over the next six years, and it will be funded in full without any increases in either borrowing or debt.

“This represents the biggest strengthening of our national defence in a generation, and as the NATO General Secretary said yesterday – it will ensure the UK remains by far the largest European defencespender in NATO, and it means we’re the second biggest NATO spender overall.

“It will provide a very significant boost to the UK’s science, innovation and manufacturing. It will make our defence industries more resilient and bigger, and it will mean we’re able to re-stock some of those global supplies required.”