Broadband provider O2 has criticised the decision to allow Everything Everywhere to launch 4G mobile broadband services using its existing 1,800MHz spectrum.
Ofcom recently gave approval for the firm - which owns T-Mobile and Orange in the UK - to launch next-generation mobile services ahead of its much-delayed spectrum auction.
Other broadband providers, including O2, Vodafone and Three, are waiting for the media regulator to auction off space in the 800MHz and 2.6GHz spectrum bands.
O2 commented: "From the very start of this process, Ofcom has said that the UK must retain a competitive market environment and that it will remove the ability for operators to behave strategically over spectrum allocation.
"To this end, Ofcom's auction proposals had much to commend them."
However, O2 said it was "concerned" that Ofcom's other proposal - to allow one operator to launch 4G early on its existing spectrum - is "contradictory to its objective" of delivering a competitive market environment with four competing players.
"This could expose the process to further risk of delay," the service provider claimed.