The proposed merger of Nielsen and the advertising arm of Ebiquity has been referred for an in-depth probe after the companies failed to satisfy concerns raised by the competition watchdog.
The UK Competition and Markets Authority (CMA) previously warned that the two firms faced limited competition if they joined forces, and asked them to provide "acceptable undertakings" to avoid a more rigorous investigation.
However, on Monday the CMA said Nielsen and Ebiquity had not provided the necessary solutions.
In a statement, the CMA said: "Nielsen and Ebiquity have not offered any undertakings and the CMA will now refer the merger for an in-depth investigation by an independent group of panel members, supported by a team of CMA staff."
The CMA, which is now headed up by former UK Treasury select committee chair Andrew Tyrie, has 24 weeks to finish its investigation of the merger.
The regulator has concluded that the two firms, which provide data on advertising, are each other's closest competitor.
Although some competitors provide intelligence on digital advertising, there are no other companies competing with Nielsen and Ebiquity for this data across all UK media, the CMA said.
Ebiquity said the two companies "will continue to work closely" with the CMA to find an appropriate resolution.