Guardian Media Group has signed a contract with Daily Mirror publisher Trinity Mirror, which will take on responsibility for print The Guardian and The Observer from early 2018.
It means the distinctive Berliner format, introduced by former editor-in-chief Alan Rusbridger in 2005, will disappear next year, the company confirmed in a statement last night.
GMG’s printing sites in Trafford and Stratford will be closed.
Katharine Viner, who succeeded Rusbridger in 2015, said about the move to tabloid: "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."
There will now be a consultation period with affected print site staff, including the ability to TUPE across to Trinity Mirror.
GMG is trying to reverse operating losses of £38m in the previous financial year and is one year into a three-year cost-cutting programme.
David Pemsel, GMG chief executive, added: "This is an important step in our three-year transformation plan. 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."