Opponents of the proposed European Union constitution are confident they will outspend supporters of the blueprint, giving Blair another headache as he tries to appeal to the public over the heads of Britain's Eurosceptic-dominated newspapers.
M&C Saatchi is the front-runner to land the account for the No campaign, which would receive generous funding from wealthy businessmen who oppose closer integration with Europe.
The agency worked for the No campaign set up to oppose entry to the single currency, which was wound down when Blair shelved a euro referendum.
The Conservative Party will have a key role in the No camp and insiders say Lord Saatchi, the party's co-chairman, is bound to have a "pivotal role" in the campaign to oppose the constitution.
The Britain in Europe group, set up to promote membership of the euro, will be revived to form the umbrella campaign for a Yes vote.
Chris Powell, a director of DDB London, was a prominent figure in Britain in Europe and the agency had been lined up to provide the advertising for a single- currency campaign. But it is likely to face competition for the Yes account from Clemmow Hornby Inge, which is said to work with the Labour Party and has Peter Mandelson, a close Blair ally and leading pro-European, on its board.
Sources say another option would be to turn to TBWA\ London, which is expected to retain the Labour account at the general election.
The No camp plans to circumvent a £5 million spending limit by launching an ad and direct mail blitz before the official campaign, which is likely to begin in the autumn of next year.
The two organisations, which will each receive £600,000 of taxpayers' money, will be allowed a free mailshot to every voter and 90-second TV broadcasts.