BT must legally separate from its Openreach network division after it failed to address competition concerns, telecoms watchdog Ofcom said.
The regulator said it was preparing to go to the European Commission to try and force BT to legally separate from Openreach after the two sides failed to come to a voluntary agreement.
Ofcom proposed that the broadband side of the company should become "distinct" from the BT Group in July but stopped short of calling for a sell-off.
The two sides had made progress but not enough to deliver the better service for phone and broadband users it wants to see, Ofcom said.
It said it remained open to BT bridging the gap between its proposal and what is required to address its "strong competition concerns".
Openreach is the division of BT that develops and maintains the UK's main telecoms network used by providers including Sky, TalkTalk, Vodafone and BT Consumer.