Harriet Harman, Labour's deputy leader, accused the Conservatives of relying on "shady money" on Tuesday as she published figures showing how reliant the Tories are on funding from unincorporated associations.
Unincorporated associations can donate money to political parties without having to declare the source of their funding in full. Individuals have to reveal their identity if they give more than £1,500 to a local party, but, if they give through an unincorporated association, they do not have to declare it unless the donation is worth more than £7,500.
Harman said the Tories had received more than £5.5m from such associations since 2010 and that, in 27 of their key seats, more than half the funding was coming from them.
Although the process is legal, it is seen as a means of exploiting a loophole in the legislation introduced by Labour to bring greater transparency to party funding. All the main parties have taken money from unincorporated associations, although it is predominantly the Tories who raise money in this way.
Harman said: "While the Tories can't beat us on the number of activists campaigning on the ground in key seats, they have a steady stream of big donors willing to bankroll their campaign -the same people they handed a tax cut to, the people who go to David Cameron's dinners.
"Hedge funds, bankers, millionaires – you can see who they are. But there's one group of people bankrolling the Tories whose identity is completely secret."
She said that, according to Electoral Commission figures, unincorporated associations gave £372,183 to the Conservative party in the most recent quarter.She also said that an analysis of Conservative association accounts showed that in the 20 most marginal Conservative-held seats, membership income fell in 14 of them (implying that membership fell too) in 2012 and 2013. Membership was going up in just a quarter of them.
"It's no wonder they need this shady money. In three-quarters of their most marginal seats, their membership is falling," she said.
A Conservative party spokesman said: "It seems Labour, a party funded by union barons who pick the candidates and the leader, are spending more time trying to invent nonsense about the Conservatives than research and develop policies for the next election.
"If they spent less time playing politics and more creating some economic policies for the future the public might not hold Mr Miliband in such poor esteem."
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