Donations to political parties fell by almost £1 million in the second quarter of 2012, official figures show.
A total of £7,873,478 in donations was reported to the Electoral Commission, down £992,312 on the figure for the first three months of the year.
The Conservatives received £3,785,579 in donations, with Labour trailing behind on £2,964,471 and the Liberal Democrats £717,797.
Labour also received £1,640,198 out of a total of £2,416,806 of public funds accepted by opposition parties.
Declarations to the watchdog showed registered political parties owed £14,585,730 in outstanding loans, a slight fall on the last quarter's figures.
Labour received £840,275 from the Unite union, the largest donation in the period from a single source.
The party also received £429,558 from the Union of Shop, Distributive and Allied Workers, and £313,863 from the GMB union.
The Conservatives' biggest donation was £512,450 from financier Michael Farmer, who was appointed co-treasurer of the party in February.
The Liberal Democrats' largest donation was £250,000 from Brompton Capital Limited.
Conservative Party co-chairman Baroness Warsi said Labour was "overwhelmingly reliant" on cash from Unite and Mr Miliband was unable to stand up to its leader Len McCluskey.
She said: "The boss of Labour's biggest donor is threatening British businesses with more strikes as his trade union continues its takeover of the Labour Party.
"With Labour still overwhelmingly reliant on union cash, it's clear Ed Miliband is too weak to stand up to Len McCluskey and the other barons who anointed him Labour leader."
Labour analysis showed that donors who attended private meals and parties at No 10 and Chequers donated almost £800,000 to the Tories.
The party's Rochdale MP Simon Danczuk said: "David Cameron's cosy dinners in Number 10 have helped the Tories rake in hundreds of thousands from millionaire donors in the last few months, while over a million pounds has flooded into Tory coffers via their exclusive 'Leader's Club' events where ministers rub shoulders with only the very biggest donors.
"With each day that passes it's clear that David Cameron's Government is completely out of touch - wining and dining the millionaires they gave a tax cut to in the Budget while millions are paying more."
A Unite spokesman said: "Trade union donations are among the cleanest money in politics.
"They are transparently given and, unlike money to the Tory Party, come from millions of ordinary working people - not the super rich.
"Voters have a choice over the kind of country they want to be. Do they want government that is funded by the people and is open and honest about this funding? Or do they want to be governed by a party that can be bought by wealthy, faceless individuals?"