ITV announced the sale of Friends Reunited in August
ITV's £25m sale of social networking website Friends Reunited has been referred to the Competition Commission.
The Office of Fair Trading (OFT) said it had asked the Commission to study the deal due to fears over its impact on the family tree research sector.
A firm called Brightsolid agreed to buy Friends Reunited in August.
Brightsolid aims to join Friend's subsidiary website Genes Reunited with its own genealogy internet pages Findmypast.com and 1911Census.com.
But the OFT said the acquisition would reduce the players in the sector to just two - Brightsolid and market leader Ancestry.co.uk.
The OFT said this "could lead to a reduction in choice and service for consumers".
Brightsolid is owned by DC Thomson, Dundee-based publisher of comics such as the Beano.
ITV and Brightsolid said they were "disappointed" by the OFT's decision.
"We believe the merger will be good for competition and good for consumers," said Brightsolid chief executive Chris van der Kuyl.
"We look forward to demonstrating that this is a lively competitive marketplace with a multitude of free and paid offerings available to consumers."