He had been at the Treasury for only seven months when Mr Javid abruptly resigned in a row over the sacking of an aide by Dominic Cummings.

Mr Javid was a vocal supporter of 41-year-old Mr Sunak from the backbenches, but when he took over one of the biggest-spending departments in Whitehall he was put on a collision course with his former underling.

“Despite the good relationship they have had in the past, Saj would always put the needs of his department before any personal history,” said one source. “He is ferocious and uncompromising when it comes to spending settlements.”

As a former Chancellor, Mr Javid also knows exactly how to play the system when it comes to getting extra money out of the Exchequer.

Mr Sunak, however, has proved that he is very much his own man. Deploying the well-worn tactic of telling a minister he had to find extra money from within his existing budget, he stood firm against the onslaught from Mr Javid, which started last week and went on over the weekend.

It means Mr Javid will now have to decide how much he is prepared to spend on keeping Covid testing free to the public, knowing that every extra pound spent on free tests will be a pound he cannot spend on clearing the gargantuan NHS waiting lists.

Once he had accepted that he would not be getting any more money from the Treasury, the row turned to how much “standby” capacity the NHS needed in case of potential future Covid waves. The Telegraph understands he insisted on maintaining a higher level of testing, genomic sequencing and surveillance than Mr Sunak regarded as necessary.

Mr Javid argued that keeping a high level of preparedness for new variants and future waves would save money in the long run by avoiding the need for lockdowns, but Mr Sunak tried to pull rank by telling him to spend the money elsewhere.

Their bruising encounter could have long-term consequences if each tries to blame the other for any cuts in NHS services or delays to dealing with the health service backlog.