The move could provoke an official protest from the Liberal Democrats, who believe it could breach electoral law. Under the rules, parties can spend £19.7 million nation-ally in the 12 months before polling day but spending in individual constituencies, including advertising, is limited to around £8,500.
Tory officials insist their plan will not break the guidelines because the posters will mainly promote the party's national campaign. They hope that their agency, Immediate Sales, a subsidiary of M&C Saatchi, will enjoy a bigger budget than the £12.7 million the party spent on ads at the last election.
Despite Labour's lead in the opinion polls, the Tories have consistently claimed they are doing better in the 160 crucial marginals that will decide the election, expected on 5 May . The Tories are believed to have an "A list" of 40 top marginal targets and to be planning a "below the radar" campaign, which will also include a direct mail blitz.
A Liberal Democrat source said: "It is a grey area, but we will be watching very carefully. If the Tories concentrate their fire on a relatively small number of seats, they could get into legal difficulties."