BT and Virgin Media have unexpectedly joined forces in a bid to dissuade Ofcom from making changes to broadband regulation.
In a letter to the media regulator - seen by the Financial Times - the former national telecoms operator and cable giant have warned against their rivals being given greater access to business networks.
The broadband giants - along with Hull-based telecoms operator KC, which has signed the letter - are concerned that Ofcom may force them to offer 'dark fibre' access to competitors.
This would allow rival service providers to lay their own cables alongside dormant lines which are already in place, and choose which lines they use on a wholesale basis.
The letter warns that "significant regulatory uncertainty" regarding dark fibre "can undermine the return on sunk investments".
The result of this, according to BT, Virgin Media and KC, could be to disincentivise future investments in UK broadband networks.
Allowing multiple operators to tamper with the physical network will also cause service faults for customers, the service providers warned.
They are said to be "very concerned" by recent calls from some operators for Ofcom to abandon the current regulatory approach.