Boris Johnson was formally asked by his then cabinet secretary to stop seeking “advice” about his financial affairs from Richard Sharp just days before he was announced as the BBC chair, according to reports.

A leaked Cabinet Office memo shows Johnson, who was prime minister at the time, was warned by Simon Case on 22 December 2020 to cease speaking to Sharp about his “personal financial matters”, the Sunday Times said.

On 6 January 2021, Sharp was announced as the BBC’s new chair. He was Johnson’s preferred candidate for the senior role.

The advice issued by Case said: “Given the imminent announcement of Richard Sharp as the new BBC chair, it is important that you no longer ask his advice about your personal financial matters.”

The Cabinet Office document was drawn up after Johnson and Sharp sought advice in early December 2020 on accepting a loan of up to £800,000 guaranteed by the Canadian multimillionaire businessman Sam Blyth.

Sharp described Blyth, who is a distant cousin of Johnson, as an “old friend”.

Case commissioned the advice on 7 December, three days after he had an unminuted one-on-one meeting with Sharp in the Cabinet Office.

Helen MacNamara, the then deputy cabinet secretary in charge of the Cabinet Office Propriety and Ethics Team (Pet), oversaw the drafting of the advice.

Sharp and the Cabinet Office say Sharp’s involvement in the loan matter ended immediately after the meeting with Case and before the Pet advice was completed.

The Sunday Times reported that Blyth and Sharp discussed how to help Johnson with his finances on two occasions: at a dinner in September 2020 and in a telephone conversation. In February 2021, Johnson secured the loan, according to his declaration in the internal register of ministers’ interests.

Sharp has acknowledged visiting Case on 4 December 2020 to discuss Blyth’s offer of financial help. He also confirmed to the publication that he spoke to Johnson about the loan “during a No 10 business meeting”.

However, he insisted he did not need to disclose this to the appointments panel or the select committee that oversaw the BBC role. Government rules state that you cannot be considered for a public appointment if “you fail to declare any conflict of interest”.

The BBC chair insisted last week that he was given the job “on merit”. However, the circumstances of that appointment are now under investigation by William Shawcross, the commissioner for public appointments.

Sharp has repeatedly said that he did not advise Johnson or have detailed knowledge of his finances. He told the BBC last week he believes he will be exonerated, insisting he would not be quitting his job at the corporation.

Sharp is set to give evidence to MPs on the digital, culture, media and sport select committee on 7 February covering issues raised at a previous hearing as well as “developments since then”.

A spokesperson for Johnson told the Sunday Times: “Richard Sharp has never given any financial advice to Boris Johnson, nor has Mr Johnson sought any financial advice from him.”

A Cabinet Office spokesperson said: “We do not comment on leaks.”