The email from Andrew Cook, chairman of engineering firm William Cook Holdings was released under Freedom of Information request from Angela Smith MP
Clive Betts MP read Cook's email in the Commons, as The Guardian reports.
"I am the largest donor to the Conservative party in Yorkshire and have been since David Cameron was elected leader. I am delighted you are back in power, albeit in coalition."
To laughter from the Labour benches, Betts continued quoting Cook to add: "I have specialist knowledge of the situation which I would like to share with you confidentially. The loan is probably unnecessary and possibly illegal under EU rules."
Smith told the Commons that Mr Cook had donated £500,000 to the Conservative party, along with the £54,000 worth of plane flights to David Cameron (who forgot his green credentials with boasts of Tory "air superiority" in the election campaign).
The loan was cancelled three weeks later. The Coalition denies that the Tory donor's lobbying influenced its decision.