Speaking in the House of Commons, Chancellor Rachel Reeves confirmed that the plan would be published before the Nato summit in Turkey, on 7 July, and said it would "involve more money, spent more effectively and meet the scale of the challenges we face".

Under Sir Keir's leadership, defence spending has increased, however on resigning as defence secretary John Healey said a planned rise to 2.68% of GDP by 2020 fell "well short" of the 3% target he argued was necessary.

There have been reports that the Ministry of Defence is asking for an extra £28bn between now and the end of the decade, but had only been offered an additional £10bn.

Sir Keir has said he has asked all government departments to cut their investment budgets to free up extra money for defence.

But there is no suggestion that the government will increase spending above what had already been set out in the investment plan seen by Healey.

Nato Secretary-General Mark Rutte has said he expects all members of the alliance to arrive at the Turkey summit with "clear, concrete and credible plans" on how they will increase their defence spending.

European countries have come under pressure to boost their budgets, following Russia's invasion of Ukraine, and the re-election of President Donald Trump, who warned allies he would not defend them unless they spent more to defend themselves.

Conservative shadow defence secretary James Cartlidge raised the subject of defence spending in the House of Commons.

"It's a simple question," he said, adding: "Who is in charge of defence in the United Kingdom at a time of war on two fronts?

"Is it the prime minister or the right honourable member for Makerfield?"

Burnham is set to receive briefings from civil servants on taking over as prime minister, as part of Sir Keir's promise to ensure "orderly" transition of power.

Downing Street has said the outgoing prime minister has authorised access talks to begin with "prospective candidates" to be the next Labour leader.