The CEOs of Sky, TalkTalk and Vodafone UK have written to Ofcom with a plan to reform Openreach, but BT said many of the suggestions were unworkable.
Jeremy Darroch, Dido Harding and Jeroen Hoencamp joined Malcolm Corbett, CEO of the Independent Networks Cooperative Association, and Chris Pateman, CEO of the Federation of Communication Services, in co-signing the letter to Ofcom CEO Sharon White.
The regulator resisted calls to separate Openreach from BT in February, as part of its Strategic Review of Digital Communications.
However, it called for BT to open its broadband network to competitors and take future decisions about budget and strategy in consultation with its peers.
The 10-point plan from the five groups recommends that Openreach is made into a legally separate company with an independent board.
An independent body should also be set up to oversee the transition and then act as an adjudicator on issues that cannot be resolved between BT Openreach and other providers, it said.
Second, Openreach would get its own finance, legal, strategy, product design and system teams that would operate separately from BT.
In turn, its budget would be ringfenced and BT would be dropped from all branding.
Third, Openreach would have an obligation to consult “industry-wide” on matters relating to strategy, plans and development of new services.
It would also be required to demonstrate “sufficient commercial rationale” before allocating resources to where small operators have already rolled out FTTH networks.
In turn, BT’s other subsidiaries would not have to rely on Openreach as its de facto supplier.
The signatories said the measures were uncontroversial, non-drastic and would deliver a “step-change in effectiveness” if implemented “in full”.
However, a BT spokesperson said: "We are in talks with Ofcom, discussing constructive ideas to further enhance the independence of Openreach from the rest of BT Group.
"Key suggestions made by other companies today seem incompatible with the legal responsibility that the Board of any listed company has when it comes to stewarding shareholders' money responsibly."
In March, Ofcom said Openreach must improve its performance in installing high-speed business lines and slash its wholesale prices.
BT has unveiled new plans to increase its FTTP network, mainly in new housing developments, high streets and business parks.