The papers will be the first UK nationals to publish in the 315x470mm format, which is used by papers including France's Le Monde, Italy's
La Repubblica and non-UK editions of the Financial Times.
Carolyn McCall, Guardian Newspapers (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 Guardian Print Centre (GPC) at Newsfax International in Bow, East London, and a Colorman at Trafford Park Printers in Manchester.
The format switch comes more than a year earlier than GNL first announced, although an earlier launch date had been expected, particularly since print tests began at GPC on 23 May.
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