The UK's competition watchdog is launching an investigation into Trinity Mirror's £126.7 million deal to buy a string of titles from Richard Desmond's media empire.
The Competition and Markets Authority (CMA) said it will weigh up whether Trinity's purchase of the brands, such as the Daily Express and the Daily Star, will substantially lessen competition in the UK media industry.
Shares in Trinity were up 1.7% in morning trading on the London Stock Exchange, with the watchdog setting a deadline of June 7 for its initial decision.
As part of the deal, the publisher of the Daily Mirror will stump up an initial £47.7 million to Northern & Shell, followed by £59 million between 2020 and 2023 and a further £20 million in shares to the privately owned firm.
It marks a major shake-up of Britain's media landscape, bringing together politically left-leaning titles like the Sunday Mirror with more right-wing publications such as the Sunday Express.
Trinity Mirror will also seize control of Daily Star Sunday, celebrity magazines New! and Star, and a 50% slice of the Irish Daily Star.
Mr Desmond acquired the Express titles in 2000 for £125 million but the industry has had to grapple with sliding advertising revenues.
Circulation of the flagship titles have also come under pressure, with the Daily Express and Daily Star falling 6.8% and 11.1% respectively in December.
Major newspaper organisations have been attempting to offset falling advertising revenues in print by raking in more money from online adverts.
The Express.co.uk and the Dailystar.co.uk recorded 280 million page views in December, with Trinity Mirror securing 649 million across its websites and apps.