BT must issue refunds for failing to provide clear information before people signed up to services from EE & Plusnet, which the firm owns.
At least 1.1 million mobile and broadband customers were not given adequate information on the price and length of contracts, as well as fees to end contracts early.
Telecoms watchdog Ofcom has fined BT £2.8m and said – in some cases – the firm “deliberately chose” not to comply with the rules.
BT has apologised and says it will “implement the remedial actions” required by Ofcom.
“We apologise for any inconvenience caused and have taken steps to proactively contact affected customers and arrange for them to receive the information and be refunded where applicable ... Read even more