Virgin Media and EE have been fined a combined £13.3 million by regulator Ofcom for overcharging nearly 500,000 phone and broadband customers who left their contracts early.
Ofcom said both companies broke consumer protection rules by failing to make clear the charges customers would have to pay if they ended their contract early and levying "excessive" fees to leave.
The watchdog's investigation into early-exit charges at the groups found about 400,000 EE customers who ended their discounted contracts early were over-billed, which saw customers end up overpaying up to £4.3 million.
It added that almost 82,000 Virgin Media customers were overcharged a total of just under £2.8 million.
As a result, Ofcom has fined BT owned EE £6.3 million and Liberty Global's Virgin Media £7 million.
Phone and broadband companies can charge customers who decide to leave their contracts early, but under Ofcom rules, those charges must be made clear to customers.
Gaucho Rasmussen, Ofcom's director of investigations and enforcement, said: "EE and Virgin Media broke our rules by overcharging people who ended their contracts early.
"Those people were left out of pocket, and the charges amounted to millions of pounds.
"That is unacceptable."