A leading internet content delivery service is investigating 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 confirmed it is looking into an issue which had a “potential impact to performance with our CDN services”.

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

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, as well as NewsChain.

Websites affected by the outage appeared to be gradually coming back online shortly before midday, but with slow loading times.

Fastly, which has been widely reported as the source of the problem, updated its service status page, saying: “The issue has been identified and a fix is being implemented.”

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.

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.