The public appointments watchdog is to investigate the selection of BBC chairman Richard Sharp amid concerns over his role in helping Boris Johnson secure a loan.

William Shawcross, the commissioner for public appointments, said he is to review the way competition for the post was run to ensure it was in compliance with Whitehall rules.

Earlier Mr Sharp announced that he was referring his appointment to the nominations committee of the BBC board following claims of a possible conflict of interest.

In a letter to shadow culture secretary Lucy Powell, who raised the case with him, Mr Shawcross said he had called for relevant papers to be sent from the Department for Digital, Culture, Media and Sport.

“The role of the commissioner is to oversee the public appointments process and ensure appointments are made fairly, openly and on merit,” he said.

“I intend to review this competition to assure myself and the public that the process was run in compliance with the Government’s governance code for public appointments.”

Mr Sharp has asked the broadcaster’s nominations committee to look into any potential conflicts of interest as questions grew over his involvement in the £800,000 loan for Mr Johnson when he was Prime Minister.

In an email to all BBC staff on Monday morning, Mr Sharp said he wanted to ensure “all the appropriate guidelines have been followed”.

He said: “We have many challenges at the BBC and I know that distractions such as this are not welcome.

“Our work at the BBC is rooted in trust. Although the appointment of the BBC chairman is solely a matter for the Government ... I want to ensure that all the appropriate guidelines have been followed within the BBC since I have joined.”

The Sunday Times reported that Mr Sharp, a former Goldman Sachs banker and Tory donor, was involved in talks to arrange the loan for Mr Johnson, underwritten by the Canadian businessman Sam Blyth, in late 2020.

The newspaper claimed Mr Blyth raised the idea of underwriting the facility with Mr Sharp who then brought the suggestion to Simon Case, the Cabinet Secretary. Mr Sharp insisted on Sunday: “There is not a conflict when I simply connected, at his request, Mr Blyth with the Cabinet Secretary and had no further involvement whatsoever.”

But Roger Mosey, one of the broadcaster’s former top news executives, said the decision to hand him the high profile role was “surprising”.

The former Editorial Director and head of TV news told Times Radio: “If you look at all the people in the whole of the UK in an appointment process, was Richard Sharp the one with the editorial and journalistic and media industry nous? I simply raise an eyebrow.

“He may have been, but I think the outcome was surprising, maybe, if you’re looking for someone who has knowledge of handling the BBC.”

Labour’s Shadow Culture Secretary Lucy Powell said the claims were “very serious”, adding she has written to Mr Shawcross asking him to look into the process which led to Mr Sharp getting the job.

She told BBC Radio 4’s Today Programme: “I have written to him because there are some very serious allegations here and real deep concerns about the recruitment process of this very important role in terms of being the chair of the BBC [who is] there to uphold the impartiality and independence of the BBC.

“Yet we find that the guy who was appointed to this job was at the same time helping to advise the then Prime Minister on a murky, grubby arrangement deal with his messy finances.”

Liberal Democrat Deputy Leader Daisy Cooper called on the Prime Minister’s new ethics adviser Sir Laurie Magnus to investigate, saying the reports of “murky financial dealings and links to top Government-appointed jobs just don’t pass the smell test”.

The BBC chairman is a role appointed by the King, on recommendation of the government, following a process which follows the official code for public appointments. Any conflicts of interest are supposed to be declared.

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

Mr Mosey added: “It would be better if the chair of the BBC were not seen as a political appointment. It would be better if they were seen as being absolutely floating above daily politics.

“And the problem for Richard Sharp is that if he intervenes quite properly about some of the times the BBC is a bit liberal, soft left, everyone’s just going to say, ‘well, that’s because you’re a Tory and you gave £400,000 to the Tory party and you had dinner with Boris Johnson’. That’s the challenge he’s going to have now.”

A spokesperson for Mr Johnson said: “Mr Johnson did indeed have dinner with Mr Sharp, whom he has known for almost 20 years, and with his cousin. So what? Big deal.”

Asked about the storm on Monday morning, Sky News reported that Mr Johnson described the claims about Mr Sharp as “absolute nonsense” adding: “Richard Sharp knows absolutely nothing about my finances.”