Internet users are facing problems accessing many websites because of a problem with Cloudflare.
The company provides internet security and other services meant to help online businesses operate smoothly.
The US firm has warned that members of the public may see "502 errors" displayed when they try to visit its clients' platforms.
The service has more than 16 million customers ranging from the chat service Discord to the dating site OKCupid.
Downdetector indicates that the problem began at around 15:00 BST.
A 502 error code signified that an internet server has received a invalid response from another server it is trying to contact.
Cloudflare says it has already taken steps to fix the issue and is currently monitoring the results.
There has been speculation that the San Francisco-based business had suffered a distributed denial of service (DDoS) attack - in which it had been flooded with traffic. But its chief executive has said the cause has yet to be determined.
The Register reported that the firm also experienced issues last week.
More to follow