Saatchi & Saatchi beat a shortlist of Beattie McGuinness Bungay, Profero and Albion to land the account.
Saatchis is understood to have clinched the business after impressing Labour with the breadth and depth of its creative ideas (including the poster ad above), its capacity as a big agency and the new management approach under the SSF chief executive, Robert Senior. The pitch was led by Senior and the creative director, Kate Stanners.
Douglas Alexander, Labour's general election co-ordinator, said: "I can confirm we have appointed Saatchi & Saatchi and we're delighted to have them on board."
Senior added: "This is a seminal moment for Saatchi & Saatchi and a seminal moment for Kate and myself. It's a real statement of intent for the agency. We have the opportunity to take the strength and conviction that Gordon Brown has shown as Prime Minister and apply our creativity to that to do the right thing for the country."
For years, the Saatchis name was synonymous with Conservatism, even though the agency actually resigned the Tory account in 1987. It created the legendary "Labour isn't working" ad that helped bring Thatcher to power in 1979, and the Conservative ad account helped turn the Saatchi brothers into household names.
There are no details yet about the timing of Labour's pre-launch ad campaign, nor the budget it will deploy, although a £2 million donation last week from Lord Sainsbury will have helped swell the Party's coffers.