The owner of the Daily Mirror has agreed to buy the company behind titles including the Daily Express, the Daily Star and magazines such as OK!
Trinity Mirror is to pay £126.7m for the publishing assets of Northern & Shell, which is chaired by Richard Desmond.
Sale talks were announced in September, but a deal between the two has been on the cards for some time.
The sale brings to an end Mr Desmond's 18 years as a UK newspaper owner.
Northern & Shell's publishing assets include the Daily Express, Sunday Express, Daily Star and Daily Star Sunday, as well as three celebrity magazines, OK!, New! and Star.
Simon Fox, chief executive of Trinity Mirror, said: "Northern & Shell's titles have a large and loyal readership, a growing digital presence and a stable revenue mix and offer an excellent fit with Trinity Mirror."
Speaking to the BBC's Today programme, he said: "It's a very wise investment. We've had plenty of time to think about this carefully."
He said the deal would lead to cost savings, as the titles could pool their resources. "For example, [instead of] sending two reporters to a football game, we can send one." Trinity Mirror expects the deal to lead to savings of £20m a year.
Mr Fox pledged that the publications would retain their identities: "The Daily Express is not going to become left-wing and the Mirror is not going to become right-wing."
Analysis: David Sillito, BBC News
Fifty years ago, the Daily Mirror and the Daily Express were Britain's two biggest papers, selling more than eight million copies a day. Today, their combined sales are less than a million and continuing to fall.
This isn't a solution to the decline of print, but a way to make the most of a declining asset until someone finds a way of making serious money from news in the internet age.
The other question is, what will happen to the journalism? The Express's front-page formula of health, weather and migrants is carefully aimed at its loyal but shrinking band of older, conservative readers. It would be a brave owner who took it in a radical new direction.
Its Euroscepticism in particular has a long history. The Express was one of the few papers to oppose joining the Common Market in the early 1970s, although it did eventually back "yes" in the 1975 referendum.
As well as the Mirror titles, Trinity Mirror also owns a string of leading local papers and is the UK's biggest regional newspaper owner.
It publishes the Daily Record, the Sunday People and more than 200 regional newspapers, including the Birmingham Post and the Manchester Evening News.
The price being paid by Trinity Mirror is only slightly above the £125m paid by Mr Desmond for the Express titles in 2000.
The newspaper industry faces many challenges, including the growing move to online readership and the dilemma between charging readers, who may then refuse to pay and get their news elsewhere for nothing, or giving away content and risk failing to cover costs through advertising.
Mr Desmond's other interests include the Health Lottery, which he has run since 2011. It is not a national lottery, but a collection of 51 local society lotteries that raise funds for health-related good causes.
He is reported to be contemplating whether to bid for the licence to run the official National Lottery.
How the circulation numbers compare (monthly, December 2017)
Daily Mirror: 581,877
Sunday Mirror: 493,069
Sunday People: 191,188
Daily Star: 391,509
Daily Star - Sunday: 239,853
Daily Express: 364,933
Sunday Express: 317,893