Everything Everywhere's (EE's) hopes of launching 4G mobile services in September have been hit as O2 gears up to launch a legal challenge.
Earlier this month, Ofcom granted permission for EE to offer super-fast mobile broadband services using its existing 1,800MHz spectrum, following a lengthy consultation process.
However, the media regulator's decision has riled a number of Orange and T-Mobile's rivals, who are concerned that the early launch of 4G will put them at a competitive disadvantage.
The likes of O2, Vodafone and Three will be forced to wait until after Ofcom's 800Mz and 2.6GHz spectrum auction before launching their own services.
The regulator has conceded that EE will enjoy a competitive advantage during the period before other operators are able to offer their own long-term evolution services.
But Ofcom claimed that any such advantage "is unlikely to result in an enduring advantage which distorts competition to the detriment of consumers".
O2's parent company Telefónica is sufficiently concerned to have written to Ofcom threatening to challenge the decision.
As reported by the Guardian, the firm gave notice that it intends to appeal against the ruling at the Competition Appeals Tribunal.
O2 could potentially seek interim relief while the case is investigated, which could prevent EE from launching next-generation mobile services in the 1,800MHz frequency band.
The news provider reports that O2 has contacted EE, asking the firm not to go ahead with its 4G launch.
A source at O2 told the Guardian that legal action "remains an option" for the company, as it bids to protect its commercial interests.
EE Chief Executive Olaf Swantee has previously hinted that his company may litigate against the 800MHz and 2.6GHz auction rules, which it is unhappy with.
And this course of action looks far more likely if O2 succeeds in blocking the imminent launch of 4G services.
"I will commit here and now to support the auction process, even though there are aspects of the auction rules that we don't like," Mr Swantee stated.
"However, and I am hoping it doesn't come to this, if there is litigation against Ofcom's ruling, we will have no choice but to review our position".