Labour is calling for an investigation after claims that the BBC chairman helped Boris Johnson arrange a guarantee on a loan of up to £800,000 weeks before he was recommended for the job by the then-prime minister.

The party has written to Parliamentary Commissioner for Standards Daniel Greenberg following a report in The Sunday Times that Tory donor Richard Sharp was involved in talks about financing Mr Johnson when he found himself in financial difficulty in late 2020.

Mr Sharp introduced multimillionaire Canadian businessman Sam Blyth, who had proposed to act as the then-PM’s guarantor for a credit facility, to the Cabinet Secretary, according to the newspaper.

The Sunday Times said Mr Johnson, Mr Sharp and Mr Blyth then had dinner at Chequers before the loan was finalised, though they denied the PM’s finances were discussed.

Mr Sharp, a former Goldman Sachs banker, was announced as the Government’s choice for the BBC role in January 2021.

A spokesperson for Mr Johnson dismissed the report as “rubbish” and insisted his financial arrangements “have been properly declared”.

“Richard Sharp has never given any financial advice to Boris Johnson, nor has Mr Johnson sought any financial advice from him,” the spokesman said.

Of Mr Johnson’s private dinner with Mr Sharp, an old friend, and Mr Blyth, who is a distant relative, the spokesman said: “So what? Big deal.”

Mr Sharp told The Sunday Times: “There is not a conflict when I simply connected, at his request, Mr Blyth with the cabinet secretary and had no further involvement whatsoever.”

A BBC spokesman said: “The BBC plays no role in the recruitment of the chair and any questions are a matter for the Government.”

In the letter to Mr Greenberg, Labour Party chair Anneliese Dodds called for an “urgent investigation” as she cited the MPs’ code of conduct that “holders of public office should not place themselves under any financial or other obligation to outside individuals or organisations that might influence them in the performance of their official duties”.

She told the standards commissioner she was concerned that Mr Johnson “may have breached this section by asking for an individual to facilitate a guarantee on a loan whom he would later appoint to a senior public role”.

“The lack of transparency around it, like that of the issue raised around Mr Blyth, may give the impression that this was a quid pro quo arrangement,” she added.

It comes after Labour demanded a probe earlier this week into reports that Mr Johnson used Mr Blyth, reportedly worth 50 million dollars, to act as a guarantor for an £800,000 credit facility.

Ms Dodds raised concerns that neither alleged arrangement was properly declared.

She said: “The financial affairs of this disgraced former Prime Minister just keep getting murkier, dragging the Conservative Party deeper into yet another quagmire of sleaze.

“Serious questions need to be asked of Johnson: why has this money never been declared, and what exactly did he promise these very generous friends in return for such lavish loans?”