The Conservatives outspent Labour by £15m last year, highlighting the party's extra financial firepower at the last election.
Accounts for all the main political parties, published yesterday by the Electoral Commission, showed the Tories spent £49,205,000 during 2010, including on the general election, while Labour spent £33,840,000 and the Liberal Democrats just under £10m.
But the extra spending came at a cost. The accounts also show that while Labour had a £3m surplus for the financial year ending in April the Conservatives are now £6m in debt. They also suggest that Labour membership has picked up after falling for many years while the party was in power. Total membership fees rose from £4.4m to £4.9m while the Tories membership income fell over the same period.
The Lib Dems – who the accounts show are £335,000 in debt – saw their membership income increase from £890,000 to £1.02m over the period.
Releasing the financial accounts for all registered political parties in the UK, the Electoral Commission cautioned the British National Party (BNP) and Christian Party that they could face substantial fines for failing to submit their accounts on time.
Peter Wardle, chief executive of the Electoral Commission, said: "The rules on party funding are intended to ensure that voters can see where political parties get their funding from, and how they spend it.
"The 2010 statements of accounts which we have published today help to provide transparency about the finances of the larger parties and their organisations, covering the period during which the campaigns for the 2010 UK general and local elections took place."
Mr Wardle criticised the failure of the BNP and Christian Party to submit their accounts to the Commission. "The majority of parties and accounting units have complied with the law by submitting their accounts on time – however, despite the guidance and advice we offer to help parties comply with the law, two parties have yet again failed to provide accounts on time," he said. "This is not acceptable."