Just 10 individuals or companies were responsible for more than half of all donations made to the EU membership referendum, according to new research.
Campaign group Transparency International has branded the situation "a dangerous place for any democracy", given the high concentration of political donations in such a select group.
The group's research also found that 95% of all the money raised to campaign in the vote came from only 100 different donors, according to available data.
More than half of these donors had previously made significant donations to political parties.
Duncan Hames, director of policy at Transparency International UK, said: "The debate around the biggest question we have faced in a generation was financed by an astonishingly small group of exceptionally wealthy donors. That's a dangerous place for any democracy.
"It illustrates the general dependency of our country's political parties on a millionaires club of some 50 donors, many of whom also sit in the House of Lords.
"Whether cash is recognised with access, influence, honours, or a seat in Parliament, this survey shows that it undermines the public's faith in our political process.
"Our country seeks to lead the world in fighting corruption yet our politics has been repeatedly dogged by corruption scandals.
"With no limits on donations, a loosely controlled arms race on election spending, and gaping holes in the reporting regime, political parties inside government and out of it need to act before trust in politics is irretrievably lost."
Seven donors gave more than £1 million to referendum campaigns, according to Electoral Commission data.
Labour peer Lord Sainsbury donated more than anyone else, giving £4.2 million to the Remain campaign.
The leading donor to the Brexit campaign was Peter Hargreaves, co-founder of financial services firm Hargreaves Landsown, who donated £3.2 million.
The third largest donor was the Leave.EU campaign group, whose formal name with Companies House is Better for the Country Ltd.
Transparency International says this campaign embodies the lack of transparency around political donations.
Leave.EU was set up little more than a year ago and has not filed any annual accounts, yet was still able to contribute £2.1 million to Leave campaigners.
Ukip donor Arron Banks is among its directors.
Transparency International's report links a lack of transparency to growing public mistrust with politics.
More than three quarters of people think wealthy individuals are using influence on government to benefit their own interests, according to a survey by the campaign group.
It suggests changes to the current system, such as capping donations and requiring companies that donate to political causes be trading in the UK.
Will Straw, director of the Remain campaign, said he did not think there was a problem with donations to referendum campaigns.
He told the BBC Radio 4 Today programme: "The question you have to ask is what are they exerting on the campaigns, and I would say not very much.
"Everybody on our side was committed to keeping Britain in the EU, encouraging Britain to vote Remain on June 23.
"I think there's more scrutiny needed on the process for general elections and parties developing manifestos.
"One party, for example - Labour - at the last election had a number of policies which would have increased taxes on the wealthy, and the wealthy gave more to the Conservative Party than to Labour.
"I would say that's a legitimate place to look."
He added there was "a legitimate claim" that there should be a cap on political donations.