Sir Keir Starmer accused Tory leadership hopefuls of plucking £200 billion of uncosted commitments from the “magic money tree”.

The Labour leader demanded that those vying to replace Boris Johnson as Prime Minister set out the cuts or borrowing they would need as they take part in an “arms race of fantasy economics”.

He accused the candidates trying to lead the Conservative Party of “desperately trying to launder its integrity” after it was “infected with the chaos virus caught from Boris Johnson”.

But Sir Keir was forced to defend allegations about his own personality as he seeks to lead the country, declaring: “The only thing that’s boring is opposition.”

Tory candidates including Cabinet ministers Liz Truss, Nadhim Zahawi and Grant Shapps, as well as former secretaries of state Jeremy Hunt and Sajid Javid, have pledged tax cuts as they seek to win the votes of Conservative MPs and party members.

But senior Conservative Robert Jenrick, who is backing former chancellor Rishi Sunak, is among those criticising promises of “fantasy tax cuts” during a strain on “credibility”.

During a speech in Gateshead, Sir Keir said there had been “more than £200 billion worth of unfunded spending commitments” from the candidates over the weekend.

“That’s more than the annual budget of the NHS, splurged on to the pages of the Sunday papers, without a word on how it’ll be paid for,” he said.

He challenged candidates to “tell us precisely where they’re getting that money from”, whether it is from cuts to public services or increases to borrowing.

“Or is it the magic money tree that they’ve suddenly discovered having arrived back down here from the moon where they’ve been for the last 12 years?” he said.

As he demands a general election, Sir Keir did not rule out tax cuts if he entered No 10.

Instead he said: “We won’t be making any unfunded commitments, everything we say will be funded and that will be set out.”

During an interview with broadcasters, he added that he wants a “fair tax system” that would “reduce the burden on working people”.

Shadow chancellor Rachel Reeves described tax-cutting plans by Tory leadership candidates as a “festival of irresponsibility” and said Nadhim Zahawi’s proposal to force Government departments to reduce costs by 20% to fund tax cuts was “grossly irresponsible”.