Mobile phone group Giffgaff has been fined £1.4m after overcharging millions of customers.
"Unacceptable" billing mistakes lead to 2.6 million people paying more than they should have, ruled regulator Ofcom.
The watchdog has now handed Giffgaff the eye-watering penalty while slamming the firm, which is owned by O2 parent Telefonica.
"Getting bills right is a basic duty for every phone company," said Gaucho Rasmussen, director of investigations and enforcement at Ofcom.
"But Giffgaff made unacceptable mistakes, leaving millions of customers out of pocket.
"This fine should serve as a warning to all communications providers: if they get bills wrong, we'll step in to protect customers."
The overpayment affected customers who bought so-called goodybag bundles.
Pre-paid goodybags contain bundles of minutes, data and texts while the customer can also buy pre-paid top-up credit.
But Ofcom found there was a delay in applying the bundles, meaning data or minutes were often coming out of pre-paid credit meaning "customers were effectively charged twice".
Ofcom started probing GiffGaff last September after it received information from the network operator.
It said the issue took place over an eight year period between 2011 and 2019.
