Virgin Media customers have been complaining of slow speeds after upgrading to the new 30Mb service.
Some Virgin Media customers who recently upgraded to the new Super Hub router have seen a drop in download speeds, especially at peak times, according to user forums.
The Super Hub is a combined router and modem designed for customers signing up to Virgin Media's fastest packages or upgrading to the new 30Mb broadband service, launched earlier this month.
One customer said on the Virgin Media forum: "The rock-steady 20Mb I had before I upgraded to 30Mb is gone."
The UK's second biggest broadband provider said that it was aware of the problem but that the issue was only affecting a small number of customers.
A Virgin Media spokesperson said: "We are aware of some issues some customers have been seeing with data throughput when using our new Super Hub modem/router. Whilst the vast majority of customers are unaffected, we are actively investigating reports of reduced performance.
"Testing is underway, and we expect to resolve the issue by a firmware update which will be automatically pushed out to Super Hub users as soon as a fix has been confirmed."