Labour received £1.3m more in donations than the Tories in the three months following the general election in May.
Statistics released by the Electoral Commission showed that Labour was given £3.72m between July and September, compared with £2.36m for the Tories. The Liberal Democrats recorded donations totalling just under £700,000.
Labour's total was boosted by a £2m donation from the steel tycoon Lakshmi Mittal. Paul Marland, the Conservatives' treasurer, gave the Conservative Party £45,000.
But the New Politics Foundation, a think-tank, warned that some major party donors could remain anonymous by giving money through "unincorporated associations".
Peter Facey, director of the foundation, warned that extremist parties could use the loophole to hide the source of donations.
He said: "The intention of the Political Parties Elections and Referendums Act was to prevent individuals from making anonymous donations, yet the loophole allowing unincorporated associations to make donations is now being stretched to breaking point."
The three main political parties were rebuked by the Electoral Commission for failing to declare donations on time. In its quarterly report, the commission said there had been late declarations totalling £49,881 by the Tories, £27,349 by the Liberal Democrats and £9,100 by Labour. The UK Independence Party made a late declaration of £20,000.Reuse content