Labour is calling for an investigation into Boris Johnson’s verbal attack on the mayor of London during a televised government press conference on coronavirus.

The prime minister accused Sadiq Khan of “blowing” Transport for London (TfL) finances through an “irresponsible fares policy”, claims that have been rejected by the Labour incumbent.

The remarks made on Monday from Downing Street’s £2.6m media briefing room came during the pre-election purdah for the London mayoral election on 6 May, which places limits on government publicity around the election.

In a letter to the cabinet secretary, Simon Case, Labour’s deputy leader accused Johnson of breaching the ministerial code in what she called a “political attack” on her party’s candidate.

Angela Rayner said the ministerial code “clearly states that official facilities and resources may not be used for the dissemination of party political material”.

Rayner said the comments were “unprompted” and “entirely unrelated” to either the press conference or to the question Johnson was asked.

She has called for an investigation into the incident and demanded Johnson issues a public apology “for misusing public resources for party political gain and misleading the public”.

Khan, who is standing for a second mayoral term as the Labour candidate, accused the Conservative leader of using his government platform to “tell lies”.

Khan 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% before the pandemic. But also he increased TfL’s debt by more than £7bn.

“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.”

Asked at Monday’s press conference about potential government support for London’s pandemic recovery, Johnson said: “As for the finances of TfL, I must respectfully remind you that I left them in robust good order. And it is not through any fault of my own that the current Labour mayor decided to blow them on an irresponsible fares policy.

“We’re doing our best to help them out and will continue to do so. But I’m afraid you’ve got to look at some of the decisions that were taken by the current Labour mayor as well.”