Virgin Media is being investigated by the telecoms regulator over complaints that it is too difficult for customers to cancel their contracts.
People told Ofcom they struggled to speak to a customer services agent by phone, with some calls being dropped and others facing long waiting times.
Others said they had to make repeated requests to cancel their services.
Virgin Media said complaints relating to "difficulties leaving" had halved over the past year.
It is the latest in a series of setbacks for the media giant, which has come under fire in recent months over disruption to its services. That led to an apology in April when thousands of people in the UK reported they were unable to access the internet twice in one day.
In June, some Virgin Media email users were left unable to send or receive emails for 36 hours, and while this issue was resolved, some customers still could not access their inboxes two weeks later.
The BBC has asked Virgin Media whether this disruption to emails has been fully resolved.
All of this comes after telecoms companies raised their prices substantially in April, with Virgin Media telling customers they faced an average 13.8% increase in their bills.
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Ofcom is also investigating how Virgin Media has handled complaints, and whether customers knew their rights.
It said companies should tell customers that they have the right to escalate their complaint to an independent ombudsman.
"Our rules are there to protect people and make sure consumers can take advantage of cheaper deals that are on offer," said Ofcom chief executive Dame Melanie Dawes.
"That's particularly important at the moment as households look for ways to keep their bills down.
"We're taking action today, on behalf of Virgin Media's customers, to investigate whether the company is putting unnecessary barriers in the way of those who want to switch away."
Ofcom said it was important for customers to be able to switch providers easily in order for the telecoms market to remain competitive.
This means people can take advantage of better deals elsewhere, and potentially save money.
If it finds Virgin Media broke the rules, the company could face a fine and be told to change its procedures.
"Any allegation that Virgin is making it tough for customers to cancel its services is a very bad look for the UK's third-biggest broadband provider, particularly in the midst of a bitter cost-of-living crisis," said Alex Tofts from comparison site Broadband Genie.
"If Ofcom's investigation finds the company in breach of its rules the damage to its reputation is likely to far outweigh any fine."
A Virgin Media spokesperson said: "We are committed to providing our customers with excellent service, supporting them with any issues and giving clear options should they wish to leave.
"Complaint rates relating to 'difficulties leaving' have halved over the past year, showing the progress we're making, and we will keep working with Ofcom throughout its investigation, while making further improvements in how we handle customer complaints to provide a better overall experience."
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Virgin Media investigated over contract cancellations
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