Ex-chief of the general staff Richard Dannatt said spending should rise to a 3.5% share of the economy.

The UK currently spends around 2.3% of gross domestic product (GDP) on defence, a figure the government wants to increase to 2.5%.

The government has promised its strategic defence review will set out the “pathway” to reach that 2.5% goal, but Dannatt said the process would be a “failure” unless spending went even higher.

“Unless Keir Starmer and (Chancellor of the Exchequer) Rachel Reeves can find ways of producing more money, well beyond 2.5% towards 3 or 3.5% for starters on our defence budget, then this strategic defence review is going to be hollow, it’s going to be a failure and, frankly, it’ll consign Keir Starmer to the bin of history,” he said.

The UK and its allies are under pressure from US President Donald Trump to do more to shoulder the burden of European defence.

Dannatt, head of the army from 2006 to 2009, told BBC Radio 4’s “Week in Westminster” that the UK military is “so run down” it could not lead a future peacekeeping mission in Ukraine.

He said around 40,000 UK troops could be needed for such a mission and the UK “just (hasn’t) got that number available.”

“Our military is so run down at the present moment, numerically and as far as capability and equipment is concerned, it would potentially be quite embarrassing.

“If we were to deploy 10,000 troops, each rotation for six months, that would effectively tie up 30,000 or 40,000 troops – and we just haven’t got that number available.

“So, there are some big issues here that today’s politicians won’t really have considered,” he said.