TM, the country’s biggest regional newspaper publisher as well as the owner of several national titles, bought Richard Desmond’s Northern & Shell titles in a £127m deal in February.
However the Competition and Markets Authority has now announced it is to investigate the deal on media plurality grounds.
The move by the CMA will prevent the two business integrating until the investigation is complete.
In a statement issued this morning, the CMA said it will “assess whether this deal could significantly reduce competition within the UK media sector, in line with its merger guidance.”
The watchdog is inviting comments on the deal from interested parties by 25 April and is due to issue an initial ruling on the takeover by 7 June.
If it decides that no serious competition concerns are raised by the takeover, it could announce at that point that it is taking no further action.
Alternatively it could launch more detailed investigation likely to last up to six months.
Trinity Mirror has so far not responded to requests for a comment on the announcement.