Political parties spent £10m less on the 2010 general election campaign than on the 2005 poll, according to figures from the Election Commission.
Campaign spending by parties in May totalled £31.1m - compared with £41.7m five years earlier, the watchdog said.
The Conservatives spent £16.6m, marginally less than their £17.8m outlay in 2005. Labour's expenditure halved from £17.9m in 2005 to £8m.
The figures do not include spending by individual candidates.
Parties contesting seats had to provide details of all items of spending, including invoices and receipts for every item worth more than £200. These have been published by the Electoral Commission.
The watchdog tracked expenditure in the year running up to the election.
A limit of £19.5m was imposed on total spending per party over the period, equivalent to £30,000 for each of the UK's 650 constituencies.
While the amount spent by the Conservatives and Labour fell, the Lib Dems increased their spending from £4.3m in 2005 to £4.7m this time around.
The SNP, the UK Independence Party and the Green Party - which gained its first parliamentary seat in May's election - all increased their outlay compared with 2005.
However, BNP spending dropped to £29,460 this year compared with £112,068 five years ago.
Separate figures have been published for parties contesting seats in Northern Ireland.
The Conservative and Unionist Party spent the most - £109,941 - of any party, followed by Sinn Fein and the Democratic Unionists which spent £64,078 and £59,086 respectively.
A breakdown of spending in eight categories shows that most resources were ploughed into campaign leaflets and other unsolicited materials.
These accounted for £12.3m of total spending, up from £8.9m in 2005.
In contrast, spending on advertising - such as billboards - fell sharply from £15m to £9m while funds spent on rallies and other events dropped from £4.1m to £1.7m.
Third party organisations spent a total of £2.2m over the election period. Both public services union Unison and anti-fascist organisation Searchlight are to be fined for not submitting returns on time.
"We first published details of campaign spending following the 2001 UK election," Peter Wardle, the Electoral Commission's chief executive said.
"Voters have become used to election spending being transparent, with details of the amount spent by each party available on our website."