The Top 100 Mailers 2010 report shows that as the political parties stepped up their marketing in advance of the General Election last May, the Lib Dems increased their year-on-year spend by more than 2000%, to £4.82m.
The report uses data supplied by The Nielsen Company covering the period from July 2009 to June 2010.
Meanwhile the Conservatives saw an increase of just over 400% on the previous year in the amount the party spend on direct mail, according to the report. A Conservative spokesman said direct mail was an "important campaigning tool" for the party.
Nielsen calculates that the Conservatives spent £5.83m on direct mail in this period.
Overall spend on direct mail by the top 100 users rose by 1.5% on the previous 12 months to £896.1m after several years of declining spend.
Retailers and telecoms brands now dominate the upper echelons of the top 100 spenders, as financial service brands retreated from the channel. Home shopping giant JD Williams was the biggest spender, increasing its spend from £20.6m to £45.9m.
Virgin Media, BT and BSkyB are among the top 10 users of direct mail.
Both the Conservatives and Lib Dems said the General Election, which was held on 1 May this year, was the catalyst for their embrace of the direct mail channel. Several Tory party mailings featured letters from party leader David Cameron to the electorate.
A spokeswoman for the Lib Dems said: "Naturally our use of direct mail goes up in an election year and local elections too and next year will be regional elections."
Direct mail and leaflets were effective for "reaching a wide audience and particularly for getting to people who don’t have access to the internet," she said.
The Top 100 Mailers 2010 report was sponsored by postal consultancy Post-Switch.
The Top 100 Mailers 2010 supplement is available with the current issue of Marketing.