An impartiality row threatens to grip Downing Street amid claims Boris Johnson used a government platform to attack a political opponent during the local election purdah period.
At a press conference on Monday, the prime minister alleged that London mayor Sadiq Khan was "blowing" Transport for London's (TfL) finances through an "irresponsible" fares policy.
Mr Johnson claimed that he had left the network's finances in "robust good order" after finishing his second term as mayor in 2016. Mr Khan has denied the claims.
The prime minister's political attack came during a Covid-19 update in Downing Street's new £2.6m publicly-funded media briefing room.
The comments were made during the pre-election purdah period for the London mayoral election on 6 May. Purdah places limits on government publicity around the election.
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The restrictions, outlined in the Local Government Act 1986, ban government officials from publishing "any material which, in whole or in part, appears to be designed to affect public support for a political party".
London Labour claimed on Tuesday that Mr Johnson's comments about Mr Khan breached these regulations and it was considering writing a formal letter of complaint to the BBC and cabinet secretary Simon Case, who is also head of the civil service.
The SNP has also claimed Mr Johnson's Covid briefings were "abusing BBC neutrality".
Launching his manifesto for a second term as mayor at a play centre in north London, Mr Khan accused the prime minister of using his government platform to "tell lies".
"If he's going to use a government platform to talk politick, he should at least tell the truth," he said.
"It is a fact that when I became mayor the TfL's deficit was more than £1.5bn. I reduced it by more than 71 per cent before the pandemic. But also he increased TfL's debt by more than £7bn.
Mr Khan added: "I think it is inappropriate for all of us to follow the rules and abide by the rules, and Boris Johnson to yet again break the rules in the way he's done.
"Firstly, during the purdah period using a government platform to attack a Labour candidate. But secondly, to tell lies."
John Nicolson, the SNP’s media spokesperson at Westminster, accused the prime minister of going on a “political rant” at Monday's press conference.
“Boris Johnson’s political rants on live television are abusing BBC neutrality. His briefings are meant to be about public health advice not party politics."
He added: “These briefings were set up to give the public impartial health advice to help keep people safe and understand the Covid restrictions.
“The PM is putting the BBC in an invidious position. But if he won’t behave it must enforce its own rules over impartiality and electoral law."
The Independent has contacted Downing Street for comment.
Additional reporting by Press Association
