In a speech as part of her party's The Choice summer campaign, Harriet Harman will say figures show that since 2010 the Tories have received £5.6 million from so-called "unincorporated associations" while their membership numbers are falling in three quarters of the Conservative Party's most marginal seats.
"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 Labour deputy leader will declare.
"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.
"Since the last election, the Tories have been given over £5.5m by unincorporated associations; organisations, who don't declare where their money comes from. They donated £372,000 in the last three months alone." The money, Ms Harman will claim, is being funnelled into the Tories' key marginal seats.
She notes how in 27 of them, more than half the money raised since 2010 has come from "these secretive clubs", which include the United & Cecil Club and the National Conservative Draws Association.
She will add: "It's no wonder they need this shady money. In three-quarters of their most marginal seats, their membership is falling.
"We may be outspent but we won't be out-organised or out-campaigned. They want to buy the election. We're fighting to win it."
In the second quarter of this year, the Conservative Party raked in £7.2m in donations, almost double the Labour Opposition's total. Some £372,183 came from unincorporated associations, the Electoral Commission figures showed.