Labour spent £80,000 more than the Conservatives running their general election campaign in May, according to official figures.
It is the first time Labour's spending exceeded their main rivals in British political history, analysts say.
Overall, three main political parties spent more than £40 million on their national campaigns - about £15 million more than 2001.
Labour paid £17,939,617 for its campaign compared to the Tories' £17,852,240, according to Electoral Commission figures. The Liberal Democrats ran a smaller campaign costing £4,324,574.
Among the smaller parties, the UK Independence party (Ukip) spent the most with £650,000. Respect came second with £321,000, the Ulster Unionist party (UUP) £251,119 and the Scottish National party (SNP) £193,987.
In the 2001 general election campaign, the Tories under William Hague spent about £12.7 million, Labour £11 million and the Lib Dems £1.4 million.
Following the unveiling of the new figures, the New Politics Network demanded that the Respect party publish its annual accounts, citing an apparent discrepancy in donations raised and actual spending.
Each party was entitled to spend up to £19,380,000 under election rules.
Running the election cost more than £90 million, according to the Department of Constitutional Affairs.