When major services like Facebook go down, the cause is often DNS related. Virgin Media confirmed in a tweet that this is indeed the case, noting that it believes the issue lies with Facebook. The provider has also displayed notices on its service status website.

"We're aware of an intermittent issue that's affected some of our broadband customers accessing Facebook and Instagram," the company said in a statement. "We believe this was an issue with Facebook's servers, which we understand to be resolved. We're contacting Facebook to confirm our customers can now access these sites."

DNS, which stands for Domain Name Server, is an internet technology that links website addresses to a corresponding server. If that link is severed then a request to visit an address like Facebook.com won't reach the intended destination.

Engineers quickly identified the issue and have been working throughout the day to restore access, but that hasn't stopped customers from venting their frustration on Twitter. Website checking service Down Detector has also collated thousands of reports originating from London, Nottingham, Norwich, Brighton and other major UK towns and cities.