Online travel website Lastminute.com lost almost a day of business last week following a power failure at its communications supplier Colt.
A power outage at a Colt datacentre brought down the main Lastminute website and subsidiary sites including Medhotels.com, Holidayautos.co.uk and Travelocity.co.uk.
Lastminute's website was offline at 3pm on 4 July until 2am the following morning.
The downtime came during one of the busiest times of the year for the travel industry.
In an average week 2.5 million people log onto Lastimute, and more than 750,000 people buy package tours on the site every year.
Colt is analysing the cause of the problem, which occurred at one of its datacentres. "This will help us put in place preventative measures to avoid any reoccurrences," said a spokeswoman.
She said the fault lay with one of the main circuit breakers. "The issue was then resolved and full power restored to customers. It affected a small number of customers housed in that data room."
A Lastminute spokeswoman said the business was working with Colt to ensure the problem didn't happen again.
The site is also one of the largest sellers of West End theatre tickets.