The regulator Ofcom ruled the company had overcharged for Ethernet services and must hand back £94.8m to communication providers BSkyB (LSE: BSY.L - news) - the owner of Sky News - Talk Talk, Virgin Media (NasdaqGS: VMED - news) , Verizon UK and Cable & Wireless.
Ethernet services are mainly used by businesses and provide dedicated broadband capacity between different locations.
Ofcom said it received the first complaint in 2010 that the charges levied by BT were "not cost orientated".
It had continued to receive related claims ahead of today's decision, the regulator stated.
BT, which said in November (Xetra: A0Z24E - news) that its second quarter revenues had been hit by a triple whammy of recession, regulation and rain, has two months to decide if it will appeal the decision.