Saatchi and Saatchi, famous for its "Labour Isn't Working" slogan that helped propel Margaret Thatcher to power in 1979, has been taken on by Labour to run its ad campaign for the next General Election.

The appointment of the agency, which for years has been associated with the Conservatives, was welcomed by Douglas Alexander, Labour's election co-ordinator, who said: "We are delighted to have them on board."

However, Westminster-watchers observed that the contract is, given the agency's history, laced with irony and will not dampen speculation that Gordon Brown is contemplating a snap autumn poll. The governing party declined to say how much the contract was and for how long it would last.

Labour released an image it said had been crucial to the agency's success, showing a suited Prime Minister accompanied by the slogan: "Not flash, just Gordon."

Saatchi and Saatchi saw off a number of other leading advertising agencies to win what is regarded as one of the most high-profile contracts in the industry.

Robert Senior, Saatchi and Saatchi's chief executive, said securing the Labour contract was a "seminal moment" for the agency, whose clients include Carlsberg lager, Ariel washing powder, Toyota cars and the NSPCC.