Labour and the Tories are expected to cut the legal campaign limit for election spending in an attempt to head off a backlash from taxpayers over the plan to increase state funding to parties to reduce their dependency on donors.
Parties are each allowed to spend up to £20 million on the election in the 12 months preceding it. David Cameron, the Conservative Party's leader, has proposed a £15 million limit for future elections.
The proposal will be considered in a review of party funding by Sir Hayden Phillips, a former top civil servant.
The Liberal Democrats also back a £15 million limit and have called for extra state funding for parties, to be funded in part by a 10 per cent cut in the Government's adspend.
Labour, Tory and Liberal Democrat officials said this week that advertising would be the most vulnerable element of their budgets if a lower limit is agreed.