But the party still raked in less in sizeable gifts than the Conservatives, by £4,116,006 to £3,136,44.
The UK Independence Party (Ukip) was buoyed by its receipts more than doubling to £160,000.
The two coalition parties also benefited from a highly unusual shared bequest of £520,000.
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.
But 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 through being in government.
It is understood that the split bequest came about because Joan Edwards specified in her will that £520,000 be donated to whichever party was in government.
The fact that the Tories and Lib Dems are sharing power caused some initial confusion, but the sum was eventually divided between them based on the number of ministers and MPs, with the larger party getting £420,000.
Conservative party chairman Grant Shapps said: "Despite Ed Miliband's promise of change, these independent figures prove his Labour party is still dominated by the trade unions. They choose the candidates, pick the leader and remain Labour's biggest donors, providing three-quarters of the party's money.
"Until Ed Miliband stops taking his union paymasters' cash, he will be too weak to stand up for hardworking people. Instead, he can only offer what the union barons want in return for their money: the same old Labour policy of more spending, more borrowing and more debt, exactly what got us into this mess in the first place."
The Lib Dems raised £801,448 from donations over the three months, down from £842,710 in the first quarter.
Labour also reduced its overdraft facilities by £2.5 million over the period and took out two new loans of £1.22 million each with the Co-operative Bank and Unity Trust Bank.
The party is listed as having outstanding loans of £12.79 million at the end of June, although that represents the initial amount that was advanced and does not reflect routine repayments since.
By contrast, the Tories are recorded as having £2.6 million of loans but have access to overdraft facilities of up to £10.13 million.
Shadow justice secretary Sadiq Khan said: "The Tories have raked in over £1 million from private dinners with David Cameron and senior ministers in the last quarter. And more than two thirds of that comes from the City - the bankers and hedge fund bosses whose taxes David Cameron cut.
"Hardworking families are seeing their living standards squeezed, with prices rising faster than wages. Meanwhile, David Cameron shows how out of touch he is, standing up for the millionaires who fund his party.
"It's nearly two months since David Cameron promised to publish the results of Lord Gold's inquiry into the Tories' dinners for donors. We're still waiting. It's time for him to come clean."