Hundreds of websites have been shut down temporarily by one of the largest web hosting companies in Britain after the personal details of customers were stolen by computer hackers.

The hackers managed to access the “master database” of Fasthosts for information, including addresses, bank details, e-mails and passwords.

The action is expected to lose vital business for hundreds of small companies in the run-up to Christmas.

Fasthosts claimed that it had no option other than to perform an emergency shutdown after it discovered that the hackers had tried to use information gleaned from its servers. New passwords had to be sent out by post rather than e-mail to avoid the information being compromised again.

Fasthosts was founded by Andrew Michael when he was still at school in Cheltenham. Mr Michael, 27, sold the business, which sells domain names and space on computer servers, to a German firm.

The company discovered a network intrusion in October and recommended that users change their passwords. Last week, staff noticed “unusual activity” on some of its sites and closed down all those that had not yet changed their passwords, as well as some that had.

Among the companies affected by the shutdown was EU Reporter, a small, web-based business that makes money from downloads and web advertising. Chris White, the owner, said that his downloads went from 47,000 a week to nothing and subscribers assumed that he had gone out of business.

He said: “The loss of readership on my site is incredibly significant to my business. I’ve lost thousands of pounds and there are literally thousands more out there like me. I’ve got a pile of letters and e-mails from long-standing customers saying they’re sorry we’ve gone bust.

“This has been a crime turned into a farce and I don’t know if we’ll survive.”

Kohul Thiagarajah, another client of Fasthosts who manages bookings for taxi companies, said: “I had my clients screaming at me for not being able to access their e-mails or their bookings.”

Barry Wise, who manages 100 sites, said: “This is worse than being hacked because I now just have to wait for the post. I can’t call them because their phone lines are overwhelmed.”

A spokesman for Fasthosts Internet Ltd said: “Last month Fasthosts wrote to all its customers to advise them that the company had discovered a network intrusion involving a Fasthosts server and, as a precautionary measure, recommended that all customers update their passwords. Fasthosts was made aware that a very small number of customers, who did not change their passwords, had experienced a compromise. As a result, Fasthosts implemented automatic password changes.

“We apologise to those customers affected for any inconvenience.”

David Roberts, the chief executive of the Corporate IT Forum, which shares expertise among its companies, said: “This could well be the biggest internet attack of its kind. The criminals could theoretically have taken everything on the database.”

Police are investigating the network intrusion.

Explore Tech & Web

• Personal Tech

• The Web

• Gadgets & Gaming