The Tories have complained to the Electoral Commission that the blitz will dwarf the official Yes and No campaigns this autumn in the North-East, North-West and Yorkshire and Humberside, which will be allowed to spend between £665,000 and £940,000.
Bernard Jenkin, the Tories' spokesman on local government, said: "This is a total misuse of taxpayers' money. The Government is spending millions of pounds publicising assemblies that nobody wants."
He added: "The truth is that £5 million is just the beginning. As with every other government scheme, the final cost of this ad campaign will be much higher than the estimate."
The Tories accuse the deputy prime minister, John Prescott (who is championing the proposed bodies), of getting round the rules restricting spending by supporters and opponents of the plan. But the Government argues it has a duty to provide neutral information telling people about the referendums before the campaigns begin.
Five agencies have been approached by COI Communications and are due to pitch next month. Defending the "Your Say" campaign, the local government minister, Nick Raynsford, said £1.1 million would be spent on broadcast media, £650,000 on posters, £475,000 in newspapers, £1.7 million on leaflets and £420,000 on events.
The campaigns are seen by the Electoral Commission as a "dry run" for the referendum on the European Union constitution.