David Cameron, Ed Miliband and Nick Clegg are expected to be dragged into the fundraising row after it emerged they all met a businessman posing as a potential donor.
The Prime Minister is reported to have been secretly filmed meeting Paul Wilmott as part of an investigation by Channel 4’s Dispatches.
The entrepreneur, who agreed to carry out the sting for the programme, was also introduced to the Labour and Liberal Democrat leaders, according to the Daily Telegraph.
The broadcast this evening is due to include footage of Mr Cameron with Mr Wilmott at a fund-raising ball, as well as an exclusive Tory donor club event at which he met a senior member of the Cabinet.
The Conservatives and Labour are insisting they have done nothing wrong and that all donations were properly declared.
The latest allegations come after Lord Strasburger resigned the Lib Dem whip pending an Electoral Commission investigation.
The peer – who denies any wrongdoing – was filmed apparently advising Mr Wilmott how to channel gifts through his “stepfather”.
Mr Clegg was seen with Lord Strasburger and the businessman at a reception in Bath.
The peer told the Lib Dem leader: “Paul is trying to find a way to support us without sticking his head too far above the parapet and we’re working out how to involve his family in making donations.”
Mr Clegg replied: “A very useful thing as well, it’s not a financial year calendar it’s an end of year calendar, so you can do things either side.
“But thank you very much for the support you give, it is much needed and will be very well used.”
According to electoral law a party must identify donors if they give a political party more than £7,500 in a calendar year.
It is illegal to attempt to deceive the commission about the source of a donation.
A Lib Dem statement said Mr Clegg had “behaved entirely correctly” and stressed that senior members of every political party attended fundraising events and assisted in raising money.
“It is perfectly normal when presented with someone who is considering making a donation to explain the rules as you state Nick Clegg did in your film,” the statement said.
The statement added that the party had run the “usual compliance checks” on the gift and concluded it was from an eligible donor.
“The Federal Party had no reason to believe that the donation was made other than by the person named on the cheque,” it said.
* Channel 4 Dispatches is being shown tonight at 8pm.