Plusnet has insisted new processes have been put in place to prevent customers from being incorrectly billed.
Following an investigation, Ofcom concluded there is reasonable grounds to believe the company did overcharge some subscribers between May 26th 2011 and September 3rd 2015.
Plusnet is now waiting to see what penalty will be handed out by the watchdog, although it has stressed that it submitted itself for investigation voluntarily after discovering the billing problems.
Speaking to ISPreview.co.uk, a spokesman also insisted that only a "very small number" of customers have been overcharged and and an apology has been issued to those affected.
"We reported it to Ofcom and contacted all affected customers so we could arrange a full refund, before the investigation had begun," the company said.
"A number of new robust measures have been implemented to prevent this from happening again."
This comes after Plusnet was identified by Ofcom as one of the most complained about broadband providers during the third quarter of 2016.
The company was the subject of 30 complaints per 100,000 subscribers, above the industry average of 20 complaints per 100,000 people.
This put it second behind BT, which was responsible for 36 complaints per 100,000 subscribers between July and September last year.