Liam Fox is facing more pressure over his role in soliciting donations used to fund close friend Adam Werritty.
Police and the Electoral Commission are being asked to investigate whether the former defence secretary and Mr Werritty broke the law.
And Foreign Secretary William Hague has signalled that the coalition will be looking at wider problems with the lobbying system in the wake of the scandal that engulfed Dr Fox.
Venture capitalist Jon Moulton is understood to have approached Cabinet Secretary Sir Gus O'Donnell to complain at being "misled" by Dr Fox, who resigned on Friday
Mr Moulton said the Cabinet minister asked him to donate to Pargav, a not-for-profit company set up by Mr Werritty. He was apparently told the firm helped provide "security and analysis", and was unaware it was funding Mr Werritty's travel to meet Dr Fox on official trips.
The millionaire has also complained to the Conservative Party about the way he was treated. There were reports that those giving money to Pargav had been promised anonymity, which could potentially be against tough rules on declaring donations.
Labour MP John Mann has written to police asking for a fraud investigation to be launched. He told the Telegraph: "Mr Werritty gave out business cards saying he was an adviser to Dr Fox. If that is not the case and he was getting money - for whatever purpose - by misrepresenting his relationship with the defence secretary, that cannot be right."
A City of London Police spokesman said: "City of London Police can confirm it has received an allegation of fraud. Officers from the force's economic crime directorate will consider the matter and establish whether or not it is appropriate to launch an investigation."
An Electoral Commission spokesman confirmed it is considering a complaint that Dr Fox had breached legislation on donations.
Meanwhile, Mr Hague dismissed as "fanciful" the idea that Mr Werritty could have been running a shadow foreign policy outside the control of government. "One adviser or non-adviser, whatever he may have been, is not able to run a totally different policy from the rest of government," Mr Hague told the BBC's Andrew Marr show.