Labour was handed more than £2.4 million by trade unions in the second quarter of the year.
However, the party still raked in less in sizeable gifts than the Conservatives - by £4,116,006 to £3,136,447.
Meanwhile, the UK Independence Party (Ukip) was buoyed by its receipts more than doubling to £160,000.
And one bequest gave an apparent endorsement of the coalition, splitting £520,000 between the Tories and Liberal Democrats.
The details emerged as the Electoral Commission released the latest figures for donations of more than £7,500.
Labour leader Ed Miliband has pledged fundamental reform of the unions' relationship with the party in the wake of the controversy over Unite's involvement in candidate selection in Falkirk.
However, the figures underline the importance of union money to the party. Unite was the biggest donor in the quarter, contributing £772,195, while the GMB gave £485,830 and Unison £458,080.
Labour also received more than £2 million in so-called short money and other funding to make up for the support that the Tories and Lib Dems get in government.
The largest gifts to the Conservatives were £280,770 and £263,600 respectively from party co-treasurers Michael Farmer and James Lupton.
It is understood that the shared bequest came about because Joan Edwards had specified in her will that £520,000 be donated to whichever party was in government. The fact that the Tories and Lib Dems were sharing power caused some initial confusion, but the sum was eventually divided between them based on the number of ministers and MPs.