Many of Virgin Media's broadband customers have been without access to several websites for a number of weeks.
Internet users have been unable to connect to popular sites such as Stack Overflow, which is a forum for computer programmers.
However, the company has not revealed the exact cause of the problem, The Register reports.
A spokeswoman told the technology news website it is aware of the situation and is liaising with the network provider to try and overcome the issue.
Mark Wilkin, Virgin Media's forum manager, went into more detail about why the lack of connectivity may have occurred.
He commented: "We've done extensive tests inside our own network and we're confident that it's an issue between us and the affected site. At that point all we can do is contact the networks down the line where the problem appears to be happening and ask them to investigate."
Mr Wilkin believes the lack of a service level agreement between Virgin Media and the network that hosts Stack Overflow may be responsible.
He added: "We're talking to Internap at the moment and we're also currently attempting to contactstackoverflow.com as well to talk to them about getting this sorted out."
A service level agreement is essentially when two network providers confirm they will pass web traffic on to one another. Internap is a leading network services firm.
In 2012, Virgin Media announced plans to double its broadband speeds for over four million users throughout the UK.
However, in November last year a number of the company's customers complained about a slowdown in its services. A statement from the company said the issue had been caused by a problem with an unnamed third-party peering network.
Earlier this year it was announced that Virgin Media had been purchased by telecommunications organisation Liberty Global in a $23.3 billion (£15 billion) deal.
The Virgin brand will be retained, but the company's chief executive, Neil Berkett, will step down once the acquisition is approved later this year.
Posted by Mark Wynn