The issue of the tax hike has been raised in particular by Red Wall Tory MPs who are concerned that they will be punished for the rise by voters at the next election.

Negotiations between Boris Johnson and Mr Sunak over the creation of the new tax were fraught with tensions, with a source telling The Mail on Sunday at the time: “The Chancellor did not want to do this. He doesn’t think we should be increasing taxes when people are struggling to get back on their feet, and particularly not when we said we wouldn’t.”

“So he has made sure he has played hardball over how the money is spent. After it comes into effect in April, it will have to cover a lot of the cost of sorting out the NHS. He is not going to just hand over endless amounts of money.”

The National Insurance rise will cost a worker on a £30,000 salary around £255 over a year, rising to £505 for anyone earning £50,000.

Waiting for Sue Gray’s report

It comes after Mr Sunak last week declined to confirm his unequivocal backing for Mr Johnson, exiting a broadcast interview as he was questioned about his support.

Mr Sunak called for patience while Sue Gray completes a Whitehall investigation into allegations of lockdown rule-breaking parties at Downing Street. The Chancellor said that he “of course” believed the Prime Minister’s account about a drinks event that took place in the Number 10 garden at the height of the first Covid lockdown in May 2020.

Asked whether Mr Johnson should resign if he lied to Parliament about the matter, Mr Sunak said he would not discuss “hypotheticals”, but added: “The ministerial code is clear on these matters.” The code states that ministers who “knowingly mislead Parliament will be expected to offer their resignation”.