BT has criticised rival broadband provider Sky for selecting a slower network repair service.
According to Ofcom documents seen by the Daily Telegraph, Sky has downgraded its contract with BT's infrastructure division Openreach so engineers are sent to customers who report faults in two days rather than one.
John Petter, Chief Executive of BT's consumer division, said this means millions of people have been downgraded to a slower repair time without being told.
He also pointed out that Sky has been a vocal critic of Openreach in recent times and the standard of service it offers.
"To be complaining about Openreach's service on the one hand and then choosing in secret to degrade the service to customers suggests a surprising level of cynicism," he commented.
"If the founders of that campaign really cared about customer service, why would they be choosing secretly to make it worse to boost their own profit?"
Sky is part of the Fix Britain's Internet campaign, along with TalkTalk, Vodafone, Three UK and the Federation of Communication Services, which is pushing for a full structural separation of BT and Openreach.
Mr Petter said this campaign has been "based around misleading claims that its authors know to be untrue, such as the lie that BT's sports business is subsidised by Openreach".
"But this latest finding shows they have sunk to a new low," he stated.
Stephen van Rooyen, Chief Executive of Sky’s UK business, has responded by insisting the company has chosen to reinvest savings from its Openreach contract into its own engineers.
He pointed out that while they are not able to repair BT's lines, they can deal with many other broadband-related issues.
"Quite simply, Openreach has failed us and our customers," Mr van Rooyen said. "We all know that there are too many late repairs, missed appointments and recurring faults.
"We therefore changed the contract, which was not being delivered, and have stepped in ourselves to provide the level of service Sky customers expect."
Mr van Rooyen went on to stress that this has required significant investment "over and above" the fees it pays to Openreach for a "sub-standard service".
This, he said, is further evidence of how the existing system does not work.
Mr van Rooyen added that Ofcom's recent customer complaints data reflected much better on Sky than it did on BT.
BT was named as the most complained about provider between April and June 2016, with a total of 26 complaints per 100,000 subscribers.
By contrast, Sky was the subject of the fewest complaints during Q2, with just five per 100,000 subscribers being received by Ofcom.
BT was ordered by Ofcom in July to give Openreach more independence and investment powers, so the broadband subsidiary could be run as a distinct and legally separate company with its own board.
However, the regulator stopped short of calling for a full structural separation, saying its proposals will provide Openreach with the greatest possible degree of independence without separating the companies entirely, and lead to decisions being taken for the good of its customers and the wider telecoms industry.