TalkTalk has raised concerns with a government plan to ensure better broadband connectivity in rural locations.
BT has offered to spend up to £600 million on upgrading 1.2 million lines that currently offer speeds of less than 10Mbps - a move that will lead to higher wholesale charges.
However, TalkTalk believes this offer is "legally questionable" and will be "more complicated and more expensive to implement than it may at first appear", the Telegraph reports.
Tristia Harrison, Chief Executive of the telecoms firm, stressed that it fully backs the government's ambition to deliver decent broadband to every home as soon as possible.
Nevertheless, she argued that without a "level playing field or properly regulated fibre prices, there is a real risk that investment in Britain's full-fibre future will be jeopardised".
Ms Harrison added that Ofcom and the government must "stand up for customers" and deliver universal high-speed broadband in a "transparent and cost-effective way that preserves competition".