Virgin Media is advising more than 800,000 customers with a specific router to change their password immediately after an investigation found hackers could gain access to it.
The company said the risk to customers with a Super Hub 2 router was 'small' but advised them to change both their network and router passwords if they were still set as the default shown on the attached sticker.
The advice followed a Which? investigation where ethical security researchers SureCloud gained access to the Super Hub 2 - although Virgin Media said the issue existed with other routers of the same age, not just their model.
A Virgin Media spokesman said: "The security of our network and of our customers is of paramount importance to us.
"We continually upgrade our systems and equipment to ensure that we meet all current industry standards.
"To the extent that technology allows this to be done, we regularly support our customers through advice and updates and offer them the chance to upgrade to a Hub 3.0 which contains additional security provisions."