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Donald Trump has attacked Elon Musk, accusing his billionaire backer of being “hostile” to his administration as a row between two of the world’s most powerful men erupted into an all-out, public feud.

Speaking in the Oval Office on Thursday, the US president said he was “very disappointed” in Musk for criticising his signature tax bill, and suggested he was trying to defend his business interests, particularly Tesla.

“He’s not the first . . . people leave my administration and some of them actually become hostile,” Trump said of Musk’s broadsides in recent days. “They leave, and they wake up in the morning, and the glamour’s gone, the whole world is different, and they become hostile.”

He added: “I’m very disappointed in Elon. I helped Elon a lot.”

Trump’s comments bring into public a deepening row with Musk that has seen the billionaire and some of his allies leave the president’s administration.

The comments came after Musk, who had hitherto refrained from attacking Trump directly, used his social media site X to suggest the president was abandoning his principles about federal spending.

Musk, who is upset that the tax bill, which is now before the Senate, would increase the US deficit by trillions of dollars, highlighted old posts by Trump in which he was critical of Republicans for raising the debt ceiling.

The billionaire also hit back at Trump’s suggestion that he had opposed the “big beautiful bill” because it axed tax credits for electric vehicles and clean energy, which have long benefited Tesla in the US.

“Keep the EV/solar incentive cuts in the bill, even though no oil & gas subsidies are touched (very unfair!!), but ditch the MOUNTAIN of DISGUSTING PORK in the bill,” Musk posted.

Musk backed Trump during last year’s presidential election, spending more than $250mn on his campaign — although the president said on Friday that he would have won the race even without the billionaire’s money.

The fracture in their relationship has now spread through Washington. Last week, Trump abruptly pulled the nomination of billionaire astronaut Jared Isaacman, a close ally Musk, to lead Nasa, ostensibly over contributions he had made to Democratic candidates in the past.

Isaacman, who was on track to receive bipartisan support from the Senate, disputed the White House’s justification for the decision.

“I don’t think the timing was much of a coincidence,” Isaacman told the All-In podcast on Wednesday. “There [were] some people that had some axes to grind, I guess, and I was a good, visible target.”

Musk had already announced that he was stepping back from his involvement in the Trump administration, where he had led the so-called Department of Government Efficiency (Doge). Musk boasted that Doge would cut trillions of dollars from federal spending, although the savings so far have been much more modest.

Steve Davis, one of Musk’s lieutenants at SpaceX who led Doge on a day-to-day basis, had also now left the administration, according to a government official.

More senior figures close to the billionaire were set to abandon the initiative in the coming days, the official said.

Musk himself has suggested that the tax bill would wipe out any savings made by Doge, which claims to have identified roughly $180bn in cuts to date. On Wednesday, the congressional fiscal watchdog said the legislation would add $2.4tn to the US debt by 2034.

On Thursday Musk rebuffed Trump’s suggestion that he had known what was in the bill and only turned on it once he had departed Washington. “False, this bill was never shown to me even once and was passed in the dead of night so fast that almost no one in Congress could even read it!”