The website of Britain's Home Office, or interior ministry, (homeoffice.gov.uk) was out of action for several hours overnight and problems continued on Sunday, when visitors found the message "page not found".

"The Home Office website was the subject of an online protest last night," a spokeswoman for the ministry said.

"There is no indication that the site was hacked and other Home Office systems were not affected. Measures put in place to protect the website meant that members of the public were unable to access the site intermittently," she said.

Twitter messages purporting to be from Anonymous said the group was behind the distributed denial-of-service attack, in which hackers flood a website with requests for information, making it unavailable to legitimate users.

The messages warned there would be further attacks on British government websites every Saturday.

Twitter messages suggested a variety of motives for the attack, including the government's plans to boost digital surveillance powers and Britain's extradition treaty with the United States.

Campaigners say the extradition rules are lopsided in Washington's favor. The highest-profile case is that of British computer hacker Gary McKinnon, who was arrested in 2002 after it was alleged that he had hacked into U.S. security systems including the Pentagon. He is still fighting extradition.

Government minister Grant Shapps said the attack on the Home Office website was "concerning". The opposition Labour Party's business spokesman, Chuka Umunna, told the BBC "we're talking about community safety issues here".

Anonymous and fellow group LulzSec have carried out a number of high-profile hacking actions against companies and institutions including the U.S. Central Intelligence Agency, Britain's Serious Organized Crime Agency, Japan's Sony Corp and Mexican government websites.

The Italian branch of Anonymous took down the Vatican's website in March, while in February, British police said they were investigating reports that Anonymous had hacked into a conference call between FBI agents and London detectives to discuss action they were taking against hacking.

(Reporting by Adrian Croft; Editing by Tim Pearce)

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