The papers will be the first UK national titles to publish in the 315x470mm format, which is used overseas by papers including France's Le Monde, Italy's La Repubblica and non-UK editions of the Financial Times.

Carolyn McCall, GNL chief executive, described the switch as "one of the most important decisions in the 184-year history of the Guardian, and the 213-year history of the Observer."

The titles will be printed on two MAN Roland Colorman presses at the specially-built Guardian Print Centre (GPC) at Newsfax International in Bow, East London, and a third Colorman at Trafford Park Printers in Manchester.

The roll-out of the format will be more than a year earlier than Guardian Newspapers initially announced, although an earlier launch date has been widely expected, particularly since print tests began at GPC on 23 May.

McCall said: "The remarkable progress we have made is testament to the professionalism and commitment of our staff, our press partner, MAN Roland, and our print partners, Trafford Park and Newsfax International."

Tom Johnson, Newsfax International managing director, said: "It's very exciting and we're looking forward to going into production later in the year."

Story by Josh Brooks