Edinburgh and Oxford Council, as well as the online home for The Traitors, were taken down, as was the Met Police website.
According to internet monitoring website Down Detector, online gaming platform Steam, Google Drive and WordPress were all down alongside a raft of local authority websites.
Error messages were displayed on the Bristol, Lambeth and Barnet council websites.
Finland has temporarily shut down the government’s public web services, while the Netherlands’ cyber-security agency said it was taking its own website offline due to the vulnerabilities.
It comes after the National Cyber Security Centre (NCSC) urged UK organisations to prepare for emergency updates to software used to run websites around the world, amid fears hackers could exploit the weaknesses to break into their systems.
Fixes were due to be released at 3pm BST on Wednesday, October 21, although they could arrive earlier.
The NCSC warned: “Where immediate remediation is not feasible, organisations should consider taking the system offline until it can be fully mitigated.”
The flaws affect PHP, software that works behind the scenes to keep websites running, including those built using popular platforms such as WordPress.
European cyber officials have warned that the weaknesses could allow hackers to run their own commands on vulnerable systems without needing to log in. Full details of the flaws and which versions are affected have yet to be published.
The warning means some website owners could face having to temporarily take their services offline while they wait for protection to be put in place.