Highlighting its concern about the impact on consumers in Aberdeen, Bury St Edmunds and Cambridge, the Competition Commission said it could order the group to sell cinemas to a rival business in those areas.
Cineworld operates 79 cinemas across the UK and has two multiplex locations in Aberdeen.
Picturehouse, which Cineworld acquired in a deal worth £47 million in December, has 21 outlets including the Belmont in Aberdeen.
Cineworld said it was disappointed with the Commission's provisional findings and added that it would continue to lobby the watchdog. A final report is due in October.