Tory leader David Cameron is facing calls to condemn his party's treasurer, Michael Spencer, after he used shares in Numis, a stockbroking firm he chairs, as security against a loan without informing the firm.
The disclosure of the deal follows a similar furore surrounding another major Tory donor, David Ross, the multi-millionaire co-founder of Carphone Warehouse.
Mr Ross quit as the firm's deputy chairman two weeks ago and also resigned his position on the board of the London Organising Committee of the 2012 Olympic Games in a blow to London Mayor Boris Johnson.
In a statement yesterday, Numis said the private investment company IPGL, in which Mr Spencer and his family hold a majority stake, used the shares as collateral.
Independent legal advice at the time the arrangement was made appeared to suggest that such a switch "did not amount to a dealing within the meaning of the AIM rules", the statement said.
"In the wake of recent press reports on this issue in relation to other companies, IPGL sought further advice and concluded that notification should now be made."
Labour former minister John Spellar said: "First of all it was their links to City short sellers; now we see that Tory donors are at the forefront of another controversial practice amongst financiers.
"How can David Cameron try and win headlines by promising to be tough on City rule-breaking when he is clearly uncritical of people who give money to the Tory Party?
"When David Ross was caught in this kind of problem, David Cameron condemned his working methods, but will he do the same for Michael Spencer, who donates even more to the Conservatives?"
The Tories made no immediate comment on the matter.