The Royal Navy Website has been taken offline after it was "compromised" by a hacker, the Ministry of Defence said today.
A Navy spokesman said no "malicious damage" had been caused by the cyber attack, which was now being investigated by security teams.
A hacker operating under the name TinKode claimed to have compromised the site on Friday.
He was also reported to have posted user names and website data on the internet.
Visitors to the Navy website this morning were greeted with the message: "Unfortunately the Royal Navy website is currently undergoing essential maintenance. Please visit again shortly."
A Royal Navy spokesman said: "We can confirm that there was a compromise of the RN web over the weekend.
"There has been no malicious damage, but as a precaution the RN website has been temporarily suspended. Security teams are investigating."
A message posted by TinKode on social networking site Twitter at 11pm on Friday read: "Minister (sic) Of Defence United Kingdom (http://www.mod.uk) - HACKED".
It gave a link to a website where the individual also claimed to have hacked US military and other official sites as well as video-sharing resource YouTube.
A Royal Navy spokesman insisted that the unauthorised access gained to the "public relations" website "did not give the hacker access to any classified information".