Last month the CMA said it was considering a probe into the deal, and this has now been put into motion.
Interested parties have until 25 April to provide the CMA with any comments.
The CMA is considering whether the merger “has resulted in a substantial lessening of competition within any market or markets in the United Kingdom for goods or services”.
Trinity Mirror agreed a near-£200m deal to acquire Northern & Shell’s publishing assets at the beginning of February. The assets include the Express and Star newspapers, weekly magazines OK! and New!, and the Westferry Printers newspaper printing site in Luton.
The expected deadline for an initial decision from the CMA is 7 June.
Trinity Mirror was issued with a “hold separate” order on 2 March, pending any CMA action. At the time the publishing group asserted that there would be no change to the editorial positions of the newspapers brands, and therefore no reduction in media plurality or detrimental impact on competition.
The company declined to make any further comment following the confirmation of the CMA investigation.
