The Competition and Markets Authority (CMA) is seeking views from interested third parties on the proposed UK merger between Three and Vodafone.
The companies agreed to a UK merger in June that would create the country’s largest mobile network , with a combined 27 million customers, leapfrogging both O2 and EE for scale.
Opinions on the mergers impact on competition in the UK are being sought ahead of a formal investigation.
Sarah Cardell, chief executive of the CMA, said: “We will be carefully considering how this deal may affect competition in the UK, which could affect the options and prices available to customers.
“We will also assess how it may affect incentives to invest in the quality of UK mobile networks.
Further opportunities to submit views will be provided once the CMA begins its formal Phase one investigation.