Three UK has thrown its weight behind calls for a structural separation of BT and Openreach.
Ofcom has ordered BT to give more independence and investment powers to Openreach, with the broadband subsidiary being run as a distinct and legally separate company with its own board.
The watchdog is hopeful its proposals will provide Openreach with the greatest possible degree of independence without separating the companies entirely, and lead to decisions being taken for the good of its customers and the wider telecoms industry.
However, the Fix Britain's Internet campaign - an industry coalition consisting of Sky, TalkTalk, Vodafone and the Federation of Communication Services - is pushing for a full structural separation and urged internet users who are unhappy with the service they receive to respond to Ofcom's consultation on the proposed Openreach reforms.
Three's endorsement comes as a major boost for the campaign, which has been criticised by BT for talking down the UK's broadband market.
The provider is arguing that mobile internet's continued success depends on a competitive broadband infrastructure market that offers independence and choice.
However, Three does not believe the system delivers this at the moment, and therefore thinks that a structural separation of BT and Openreach will create "genuine competition and prevent BT from favouring itself".
David Dyson, Chief Executive of Three UK, commented: "We have always focussed on providing the best mobile data experience and we are passionate about ensuring everyone has access to a high-quality internet connection. "Ofcom should play a crucial role in ensuring UK consumers fully benefit from the digital revolution, yet Ofcom has allowed BT to accumulate close to 50 per cent of the airwaves that are key to the mobile internet, as well as dominating the fixed broadband market."
Mr Dyson stated that consumers want "genuine choice" and must therefore make this clear to Ofcom.
This, he said, could prompt the regulator into making "bold decisions on issues like Openreach, spectrum and switching in the face of huge pressure from big incumbents to keep the status quo".