By Edith Hancock

The U.K.'s competition watchdog said it would fast-track an in-depth investigation into Nexfibre's acquisition of Netomnia parent Substantial Group, which was valued at 2 billion pounds ($2.65 billion) including debt.

"We requested a fast-track to Phase 2 to get to the right answer faster; ensuring due process, while recognizing urgency," Rajiv Datta, Nexfibre's chief executive, said. The company looks forward to engaging constructively with the regulator, Datta said.

The CMA said it set a Dec. 15 deadline to wrap up its probe into the deal. Moving to an in-depth probe isn't a finding of competition issues, it added.

Nexfibre--a broadband joint venture between telecommunication companies Liberty Global and Telefonica as well as InfraVia Capital Partners--announced the agreement to buy the U.K.'s second-largest so-called alt-net fiber network from Advencap, DigitalBridge and Soho Square Capital in February.

Nexfibre CEO Datta said at the time the deal would create the country's largest alternative fiber platform and establish a foundation for consolidation in the market.

Nexfibre plans to sell Substantial's retail business, namely its YouFibre and Brsk brands, to Liberty Global and Telefonica-owned Virgin Media O2 for 150 million pounds after the takeover and keep wholesale full fiber operator Netomnia.

Write to Edith Hancock at edith.hancock@wsj.com