Ofcom has detailed proposed price controls on BT's leased telecoms lines, saying they would mean significant reductions for many businesses reliant on the high-speed lines.
BT has "significant power" in the £2bn leased lines market, which provides vital high-speed broadband and mobile services, according to Ofcom who want to implement charge controls.
In a statement, the regulator explained: "Ofcom is proposing a form of charge control that aims to bring prices down to costs over a three-year period. This type of control, which is linked to inflation based on the consumer price index (CPI), provides an incentive for BT to make efficiency gains."
BT's traditional interface services with bandwidths up to 8mb per second will be capped at between 6.25 and 14.25 per cent of the consumer prices index, while ethernet services up to 1gb per second will be capped between 9.75 and 17.75 per cent.
Under the plans, BT will be forced to open its "dark fibre" optic lines to rivals - allowing them to take direct control of the connection. Ofcom will take responses to its proposals into account before making a final decision on implementing price controls early next year.
BT's share price is up 1.47 per cent, building on a 3.18 per cent since it revealed details of its European football offering on Tuesday.