Allegations that a wealthy Saudi businessman was offered help to secure an honour and British citizenship after donating to Prince Charles’s charities have been reported to the police and should be the subject of an investigation, a former government minister has said.

Michael Fawcett, a former assistant valet to Charles, has temporarily stepped down as chief executive of the Prince’s Foundation after claims by Sunday newspapers he helped coordinate an honour relating to Dr Mahfouz Marei Mubarak bin Mahfouz.

Mahfouz, who received a CBE in 2016, donated GBP1.5m to restoration projects close to Charles, including Dumfries House, where a garden and fountain are named after the prince, and Castle of Mey in Scotland, where a wood bears his name, the Sunday Times reported, adding Mahfouz had denied any wrongdoing.

Norman Baker, a former Liberal Democrat minister, author and critic of the royal family, has written to the Metropolitan police commissioner, Cressida Dick, urging a police investigation into the affair.

Baker said a letter, published by the Mail on Sunday, allegedly from Fawcett to an aide at the Mahfouz Foundation, appeared to be “prima facie” evidence of an offence.

In the 2017 letter, Fawcett allegedly wrote that in the light of Mahfouz’s ongoing generosity “we are willing and happy to support and contribute to the application for citizenship,” the Mail on Sunday reported.

It continued they were willing also to seek upgrade the CBE to KBE.

Baker said he had written to Dick “to say that in the light of the full transcript of the letter, there appears to me to be prima facie evidence that an offence has been committed under the Honours [prevention of abuses] Act 1925.”

“If politicians had engaged in selling honours, or in offering support for citizenship, they would be in big trouble. And the same thing should apply to Prince Charles,” he said.

Baker believes any investigation should examine what Prince Charles was aware of at the time.

The Times reported that Charles was “100%” behind the offer to help the Saudi billionaire, according to a fixer, described by the newspaper as a paid adviser to Mahfouz, and the royal had met Mahfouz in Riyadh, Clarence House, and at Dumfries House, in 2014 and 2015.

Baker said he believed the questions lead directly to Charles. “Of course,” he said, describing Charles and Fawcett as “Tweedledum and Tweedledee”.

“[Charles] can’t do without him. {Fawcett’s] already had to resign twice, and Charles has brought him back under cover of darkness. That’s what he’ll do,” he said.

“I have suggested to Dame Cressida Dick that there is good evidence, and that there ought to be an investigation based on what is in that letter. On the face of it, there’s evidence of corruption. I believe in propriety in public life. And Charles is in public office,”

The prince gave Mahfouz, 51, his “honorary” CBE – for those who are not British or Commonwealth nationals – at a private ceremony at Buckingham Palace in November 2016.

Fawcett, who in 2003 was cleared of financial misconduct allegations over the selling of royal gifts, was appointed chief executive of the Prince’s Foundation in 2018 after a reorganisation of Charles’s charities.

Fawcett has offered to step down temporarily as chief executive of the Prince’s Foundation while the trustees’ conduct an investigation.

A Clarence House spokesman said: “It is right that the Prince’s Foundation is taking the investigation so seriously and we will not be commenting further whilst it is ongoing.”

Fawcett began his royal service in 1981 as a footman to the Queen, before rising through the ranks.