A renowned fact-checking organisation has accused Rishi Sunak of "damaging democracy" and "spreading misinformation" by repeating false claims about Labour's immigration policy.
The charity FullFact said claims made by the prime minister about the opposition's position to seek an asylum returns agreement with the EU were "not based on on reliable information".
Mr Sunak and his party chair Greg Hands had both claimed Labour wanted to open the door to another 100,000 migrants a year – a claim which the charity says does not stand up to scrutiny.
FullFact had already warned Mr Sunak in September that the claim "was not based on reliable information" and urged him to stop repeating it.
But the PM ignored the warning and has since repeated it again, as has Tory party chair Greg Hands.
"As elected representatives, you have a duty to voters to lead with honesty. Any claims you make should be based on reliable information," the charity's chief executive said in a letter released on Tuesday.
"With a general election on the horizon, we want to be reassured that you will do everything possible not to mislead voters."
They added: "Spreading misinformation about political opponents is damaging to our democracy and it is important that you stop repeating this claim."
The 100,000 claim is based on a pledge by Keir Starmer that he will seek a returns agreement with the European Union for asylum seekers arriving in the UK.
But FullFact said it was "based on a number of assumptions about a hypothetical returns agreement" and included "a misrepresentation".
More than 2,700 people have contacted the PM to urge him not to repeat the claim.
In his latest letter, Andrew Dudfield, the interim chief executive of Full Fact said: "It is vital that political parties ensure claims they make about their opponents are accurate and based on reliable information, particularly when claims such as this have the potential to affect people’s opinions of individuals and parties, and how they choose to vote.
"The public are deeply concerned about untruthful behaviours from politicians, as are we. We would be grateful if you’d let us know what action you take in relation to our request."
Opposition leader Sir Keir has previously said is prepared to do a deal which would involve the UK taking asylum seekers who arrive in the bloc, in exchange for the ability to return people crossing the English Channel.
Sir Keir Starmer’s has said a Labour government would seek returns agreement with the EU— (Getty Images)
But shadow home secretary Yvette Cooper has said Labour would not join a formal European quota scheme.
Shadow immigration minister Stephen Kinnock said Mr Hands and the PM were “plucking numbers out of the air” to “divert the blame away from their own shambolic handling of the asylum system”.
“Labour is committed to stopping the small boats crossings, smashing the criminal gangs, ending £8m-per-day asylum hotel use and fixing the broken asylum system,” he added.
Mr Kinnock said: “One hundred thousand people have crossed the Channel on small boats under the Tories. Meanwhile the asylum backlog has rocketed nine-fold to 176,000 since Labour left office in 2010, despite the constant lies from Tory ministers.”
A Tory spokesman pointed to Sir Keir saying “in an on-the-record interview” that he was open to accepting quotas of migrants as part of an arrangement with Brussels.
The spokesman said 100,000 “is a conservative estimate of how many the UK would be required to take under any such agreement”.
They added: “The fact he is now refusing to engage in the substance of this discussion shows he will always prioritise scoring a quick political point over the long-term interests of the country.”
