The Conservative Party has received donations of more than £83 million from people on the Sunday Times Rich List, according to a trade union study.
Research for the GMB showed that 248 out of the top 1,000 in the list were Tory supporters who have financially backed the party.
The information, drawn from the Electoral Commission and the Register of Members' Interests from 2001 to this year, was presented to delegates at the GMB annual conference in Brighton today.
GMB general secretary Paul Kenny said: "It is clear that the wealthy look to the Tory Party to protect their interests and they have been repaid with policies like the change in income tax, down from 50p to 45p. This is not philanthropy. It is an investment by an elite in an elite to look after their interests.
"These revelations that the Tory Party is funded to such an extent by the rich and elite of our country is a shocking indictment of the real allegiance of the Tory Party.
"Two hundred and forty-eight of the country's super-rich, most of them worth hundreds of millions of pounds, ranging from the aristocracy to hedge fund managers with caravan park owners thrown in, have been quietly bankrolling the Tory Party, raising serious questions in whose interest they govern.
"At a time when working families are being hammered, it is sickening to see the vested interests of these multimillionaires getting tax breaks."