Historically, the COI has appointed agencies by medium to handle the government and public sector bodies' £251m annual spend, but the four-year deal with M4C will cover all six media channels, including print media, outdoor, online, radio, tv and cinema.

The COI could not give a breakdown by media for the annual spend, although it said that its latest annual report and accounts showed that government departments and public sector bodies had spent £40m on online advertising and £211m on other media in 2008/09.

M4C is part of media-buying network Group M – a subsidiary of global ad giant WPP.

COI deputy chief executive Peter Buchanan said: "This has been the largest competition of its kind ever to take place in the UK. The responsibility to come up with the right decision has been considerable."

The appointment is to be formally made 15 February, and the contract is due to start in April.