The Guardian and its sister Sunday title The Observer are to become tabloids, it has been announced.
The papers will switch from a mid-size Berliner format in early 2018, 12 years after changing from broadsheets.
It is part of a "transformation programme" aimed at seeing the loss-making papers break even by 2019, publisher Guardian Media Group said.
The move will also see the closure of printing sites in Trafford, Manchester, and Stratford, east London.
Printing will be outsourced to presses run by Trinity Mirror, the owner of the Mirror, Sunday Mirror and People tabloids.
GMG said its advertising business would be restructured and its cost base reduced.
A consultation with print workers affected by the change has been launched.
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Katharine Viner, Guardian editor and editor-in-chief of Guardian News & Media said: "The Berliner is a beautiful format which has served our readers brilliantly for 12 years but we know that it is our award-winning, quality, independent journalism that our readers value most, rather than the shape or size of the newspapers.
"We are going to create a new look tabloid Guardian and Observer that are bold, striking and beautiful."
The Berliner format was introduced by Ms Viner's predecessor, Alan Rusbridger, who stepped down in 2015 after 20 years in charge.
Although it is a popular size for titles in Europe, it had not been adopted by any national newspaper in the UK.Image copyright Getty Images Image caption The papers moved from Farringdon to a new site in King's Cross in 2008
GMG spent £50m on new presses for the September 2005 launch, which also saw a complete redesign of the papers.
The Guardian's masthead which had been used since 1988 was replaced by a three tone blue and white logo, and new typefaces and layout were introduced.
David Pemsel, chief executive of GMG, said: "More people are reading and supporting our journalism than ever before, but the print industry continues to evolve, and we must evolve with it."