David Edmonds, the telecoms regulator, has accused BT of "anti-competitive" behaviour and warned that he is watching the company "like a hawk".
The rare outburst comes as Oftel is in the middle of two major investigations into claims that the telecoms giant had deliberately used its monopoly position to squeeze the competition.
Mr Edmonds told The Independent on Sunday: "BT is seeing a significant risk to its market share. In some areas it is digging in ... and on occasions it has stepped over the line. BT will fight back. My job is to ensure BT fights back fairly. I will watch it like a hawk. There will be a number of skirmishes."
Oftel is investigating claims that BT "hijacked" the phone book by placing an advert for its new directory enquiries service on the cover. The directory enquiries market is being opened up to competition, but BT only allowed rivals to advertise on a sheet inserted in the phone book.
Oftel has ordered BT to stop distributing the new phone book while it investigates the issue. But Mr Edmonds said: "BT's attempt, in my view, was anti-competitive."
The other Oftel probe concerns claims that BT had switched back customers who had defected to rival companies without their consent.
A draft Oftel ruling in favour of the complainants last month incensed BT, which claimed it was "fundamentally flawed and based on out-of- date facts". BT has hinted that it might challenge the ruling through the courts if it still stands when Oftel publishes its final decision within the next three weeks.
But Mr Edmonds dismissed this as "a red herring". He added: "How many battles has BT won against the regulator? Answer: zero."Reuse content