Telecoms regulator Ofcom has ordered BT to legally separate from its Openreach division, which runs the UK's broadband infrastructure.
Openreach should become a distinct company within the BT group, the regulator said.
BT had not voluntarily addressed competition concerns Ofcom laid out in July, it said.
However, BT said its proposals were "fair and sustainable", and that it would continue negotiating with Ofcom.
The regulator said it was preparing a formal notification to the European Commission to start the separation process.
Ofcom has resisted calls to split Openreach off entirely, which telecoms rivals have sought.
Ofcom said BT had not gone far enough to address its concerns about BT's ability to favour its retail business when making investment decisions in Openreach.