The left-leaning daily and its sister Sunday paper The Observer will downsize from their mid-size Berliner format, 12 years after a reported £80 million was spent on making the change from broadsheets.
The heavily trailed change, due to come into effect in early 2018, will result in the closure of Guardian Media Group’s printing sites in Trafford, Manchester, and Stratford in east London, the firm said.
The printing will instead be outsourced to presses run by Trinity Mirror, owner of the Mirror, Sunday Mirror and People tabloids.
Katharine Viner, the 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 - and which still contain the agenda-setting journalism for which we’re renowned.”
GMG said the three-year programme, which the change was part of, would also see the publisher restructure the Guardian’s advertising business and reduce its cost base.
It said the aim was the make sure the loss-making newspaper breaks even by 2019.
It has launched a consultation with print workers affected by the closures.
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.
“We plan to continue the Guardian’s record of producing bold, brilliantly designed award-winning print journalism.
“This will impact a number of our print site colleagues and we will honour our commitments to them as part of the transition process to Trinity Mirror.”
The Berliner format was the brainchild of Ms Viner’s predecessor, Alan Rusbridger, who stepped down in 2015 after 20 years.
It is a size popular on the continent but had not been used by any national newspaper in the UK. GMG spent £50 million on new presses for the 2005 launch.
At the time Mr Rusbridger declared broadsheet newspapers “dead”, saying people increasingly found them inconvenient.
The 2005 relaunch also saw a complete redesign. Gone was the iconic masthead which had topped the paper since 1988. It was replaced by a softer, three tone blue and white logo. Typefaces, headlines and layout were changed throughout.
The paper also became the first UK national to be printed exclusively in full colour.