Nearly half of Britain's top hedge fund managers are helping to bankroll the Tories, according to a Labour analysis.
Some 27 of the 59 wealthiest asset managers included on the most recent Sunday Times Rich List rankings had made donations to the Conservatives - totalling £19 million.
The party has received £10m from the individuals since the last general election, according to the figures compiled by the Opposition.
Similar analysis by the Financial Times found that the number of City figures backing the Tories has doubled during this parliament compared with the period 2005 to 2010.
The FT found that 35% of all party funding comes from eight of the top 20 donors, all of whom have a City background.
In the previous parliament there were four City backers in the Tories' top 20 donors list who gave £8.3 million.
During Prime Minister's Questions yesterday, Labour leader Ed Miliband linked donations to the Tories to the introduction in 2013 of a stamp duty exemption on stock market transactions
Lucy Powell, vice-chairwoman of Labour's general election campaign, said: "David Cameron is debasing political debate by refusing to answer questions over his hedge fund backers. The Tories are standing up for those at the top rather than hard-working families, and David Cameron doesn't want to admit it."