The Labour Party received an income of more than £51m in 2015, nearly £10m more than the Conservatives, according to the latest party funding figures.
The Unite union was the party's biggest backer last year, donating £6.4m, although Labour's income also included public funding of nearly £8m.
The Conservatives received £41.9m, the Lib Dems £7.9m and the SNP £6m.
Parties must report donations of more than £7,500 to the Electoral Commission under party funding rules.
The latest figures mark a record year for the Labour Party, up on its 2014 income of £39.6m. It also spent £46,789,000 - the highest amount since the 2005 general election.
In its submission, the party said it had been a "hugely positive financial period" for Labour and said the "main driver" of the boost in its income was a "growth in donations from all sources: members, online supporters, affiliated unions and high value gifts".
While large sums were donated by unions including the GMB, Unison and USDAW, hundreds of thousands of pounds were also received in donations from individuals such as Hull City chairman Assem Allam and property developer Sir David Garrad.
The Conservatives spent nearly as much as they took during the 2015 general election year - £41,030,000.
Their largest donors included Addison Lee founder John Griffin, who gave large donations totalling more than £900,000 and Trailfinders founder Michael Gooley who gave £500,000.
The Lib Dems spent more than they earned last year - £9.5m on an income of 7.9m - as did UKIP, which raised £5.8m and spent £6.7m.
Thursday's publication is of figures from political parties which raised more than £250,000 for the year ending 31 December 2015.