HSBC, ITV and Waitrose websites were among those hit by a widespread blackout that briefly disrupted Thursday afternoon.
Major online banking services, including Barclays, TSB, Bank of Scotland, Tesco Bank and Sainsbury’s Bank, have been totally or partially inaccessible for a short time.
Airbnb, PlayStation Network, and Steam also experienced the temporary issue, with some showing users a DNS error.
Unconfirmed reports suggest the issue may be related to a performance product offered by Akamai, a US company, which began investigating the issues just after 5 p.m. UK time.
Its Edge DNS service is designed to improve load times and combat attacks, using a network of servers spread around the world.
Akamai later tweeted, “We have implemented a fix for this issue, and based on current observations, the service is resuming normal operations.
“We continue to monitor the situation and can confirm that this is not a cyber attack on the Akamai platform. “
The incident follows a similar outage six weeks ago, which affected prominent websites including gov.uk.
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 updated their settings.