A leading internet content delivery service has identified the source of an outage on its platform which may have caused a worldwide internet issue and taken dozens of major websites offline.

US firm Fastly, a content delivery network (CDN) which helps websites speed up loading times and present their content to users, has said a fix “is being implemented”.

Websites affected by the outage appear to be gradually coming back online, but loading times are slow.

The UK Government’s website was among a number giving error messages on Tuesday morning in what appeared to be a worldwide internet issue.

“We are aware of the issues with http://GOV.UK which means that users may not be able to access the site. This is a wider issue affecting a number of other non-government sites,” the Government Digital Service tweeted.

“We are investigating this as a matter of urgency.”

Websites for several leading media companies – including the Guardian, Financial Times, Independent and the New York Times – were also down.

Anyone attempting to access the gov.uk site was given the message: “Error 503 Service Unavailable.”

The Guardian tweeted: “The Guardian’s website and app are currently being affected by a wider internet outage and will be back as soon as possible.”

Other websites affected included the online discussion platform Reddit, the Evening Standard and French newspaper Le Monde.

A number of the affected websites had earlier begun to confirm that the issue was linked to Fastly.

Alex Hern, the Guardian’s technology editor, tweeted that Fastly “has been identified as the cause of the problem”.

“The outage, which began shortly before 11am UK time, saw visitors to a vast array of sites receive error messages including ‘Error 503 Service Unavailable’ and a terse ‘connection failure’,” he tweeted.

The Quartz site has also attributed the issue to Fastly via Twitter.

Given the nature of this kind of technology, it's probably not a software issue. It's more likely to be a physical issue or a hardware failure somewhere

Fastly offers services such as speeding up loading times for websites, protecting them from denial-of-service cyberattacks and helping them deal with bursts of traffic in order to stay online and stable.

A software testing expert told the PA news agency that the issue was probably due to a physical problem rather than software-related.

“Given the nature of this kind of technology, it’s probably not a software issue,” Adam Leon Smith, from BCS, The Chartered Institute for IT, said.

“It’s more likely to be a physical issue or a hardware failure somewhere.”

He added: “Some of the websites will have removed their dependency on Fastly in order to get themselves back online and so I think within a few hours things should be ticking along nicely again.”