Prime Minister Sir Keir Starmer has said he regrets saying the UK risked becoming "an island of strangers" in a speech about immigration.
The prime minister was accused by some critics of using divisive language when he made a speech to announce plans to cut immigration in May.
Some MPs compared Sir Keirs remarks to those made by former MP Enoch Powell in a well-known speech about immigration in 1968.
Downing Street rejected the comparison and said the PM stood by his words.
But the PMs comments have now been met with fury by some of those loyal to him, who have called them "weak" and "outrageous".
In an interview with his biographer, Tom Baldwin, Sir Keir said: "I wouldnt have used those words if I had known they were, or even would be interpreted as an echo of Powell.
"I had no idea – and my speechwriters didnt know either.
"But that particular phrase – no – it wasnt right. Ill give you the honest truth: I deeply regret using it."
The interview was published in the Observer newspaper ahead of Sir Keirs one-year anniversary since becoming prime minister next week.
Sir Keirs comments suggest neither he nor his speechwriters were aware of any similarity to a line in Powells 1968 speech.
In that speech, Powell described a future in which Britons "found themselves made strangers in their own country".
It is widely known as the Rivers of Blood speech because of Powells reference to "the River Tiber foaming with much blood", when setting out his fears about immigration.
The newspaper interview, in which he repudiates not just the speech, but much else of the political strategy of his first year in office, is being met with total fury by Sir Keirs loyalists.
"Outrageous", "weak", and "totally lacking in moral fibre" were some of the words used by those allies of the prime minister.
There is particular anger at the perception that he is throwing his closest aides under a bus.
A senior government source said they were too angry to speak about it.
In the Observer interview, Sir Keir said he made the speech on immigration not long after an alleged arson attack on his family home in London.
"Its fair to say I wasnt in the best state to make a big speech," Sir Keir said. "I was really, really worried."
He said his wife Victoria was "really shaken up", adding "it was just a case of reading the words out and getting through it somehow".
The Observer article quotes Sir Keir as saying he should have read through the speech properly and "held it up to the light a bit more".
Responding to the interview, Reform UK leader Nigel Farage said the PMs admission of regret at using the phrase was "absolute proof that Keir Starmer has no beliefs, no principles and just reads from a script".
"This country needs a leader who has vision," Farage posted.
The comparison to Powell was made by John McDonnell, who lost the Labour whip last year after rebelling on a welfare vote.
Speaking in Parliament in May, the independent MP said: "When legislation of this nature is being introduced that is serious and could be contentious, its critically important that ministers use careful language.
Questioning Home Secretary Yvette Cooper, McDonnell said: "When the prime minister referred to... an island of strangers, reflecting the language of Enoch Powell, does she realise how shockingly divisive that could be?"
In the same debate, Labour MP Nadia Whittome said immigrants were being "scapegoated for problems that they didnt cause" and that "the rhetoric surrounding this" risked stirring racial abuse.
The MP asked: "Why are we trying to ape Reform, when that will do nothing to improve our constituents lives and just stoke more division?"
Yvette Cooper later defended the language used by Sir Keir, saying his speech was "completely different" to Powells and telling the BBC: "I dont think its right to make those comparisons.
"The prime minster said yesterday, I think almost in the same breath, talked about the diverse country that we are and that being part of our strength.
"I know that everybody always gets caught up in focusing on different phrases and so on, but we do have to be talking about the policies."
The launch of the governments immigration plan in May followed local elections in England earlier this month that saw Labour lose the Runcorn and Helsby parliamentary seat to Reform UK.