The sites HSBC Web, ITV and Waitrose were among those hit by a widespread blackout that briefly caused disruption Thursday afternoon.
Major online banking - including Barclays, TSB, Bank of Scotland, Tesco Bank and Sainsbury 's Bank - were wholly or partially inaccessible for a short time.
Airbnb, on PlayStation Network and Steam also encountered the temporary issue, with some telling users an error DNS.
Unconfirmed reports suggest the issue may be related to a performance product offered by Akamai, a US company, which has started investigating the issues. just after 5pm UK time.
Its Edge DNS service is designed to improve load times and combat attacks, using a network of servers spread across worldwide.
Later, Akamai tweeted, "We have implemented a fix for this issue, and based on current observations, the service resumes normal activities.
"We are continuing to monitor the situation and can confirm that this is not a cyber attack on the platform - Akamai shape. "
The incident follows a similar outage six weeks ago, which affected prominent websites including gov.uk.
The US company Fastly said the problems on June 8 were due to an "undiscovered software bug" in its system, which was triggered by a single anonymous customer who put update its settings.