Mr Johnson publicly watered down his position on the increase for a few days in late January. When asked if the increase would go ahead, he only said that the money for healthcare must be found.

In the end, Mr Johnson and Mr Sunak put their names to a joint article in The Sunday Times, confirming the increase would happen.

Before that point, Mr Sunak was frustrated that Mr Johnson was considering backing out of the tax rise they had agreed in the autumn, according to allies of the Chancellor.

One said the Chancellor was arguing: “This is not serious government.”

Tory MPs continue to push for the National Insurance rise, which kicks in this April, to be scrapped amid a cost of living crisis. It will be announced in the Spring Statement next Wednesday.

But Number 10 and Number 11 press spokesmen have repeatedly denied any change is imminent, arguing it is too late given that the tax rise is written into law and the money is needed for healthcare.

Mr Sunak, speaking at the Tory Party spring conference on Friday, said increasing taxes was the last thing he wanted to do, but stressed the need for sound finance.

“We’ve made the difficult decisions that we have to make,” Mr Sunak told the conference in Blackpool.

“My priority going forward is to cut taxes. I made that very clear at the Budget.”

However, no date was given for when tax cuts will start.