The Royal Navy's website has been "temporarily suspended" after a hacker gained access to site and subsequently posted site administrator's usernames and passwords online.
Romanian hacker, TinKode, is thought to be behind the SQL injection attack, after he claimed responsibility on his Twitter page on November 5 and then posted a link to the webpage detailing the user names and passwords.
However, a spokesman for the Royal Navy said: "There has been no malicious damage" and said that the information that was leaked was not confidential.
Graham Cluley, senior technology consultant at security firm Sophos, said: "The MoD is lucky that on this occasion, those behind the hack have been motivated more by mischief than malice. With luck this security breach is more of an embarrassment to the Ministry of Defence than a more significant assault on a website presenting the public face of an important part of the armed forces."
Cluley added that: "All website owners should take note of this attack and the need to build secure websites that cannot be breached easily by hackers. The Royal Navy could have found itself in a far more sinister situation if hackers had chosen to embed spyware onto the website and infected visitors' computers to steal classified information. Owners of other websites shouldn't be under the misapprehension that it couldn't also happen to them."
According to Zone-h, Tinkode has defaced 52 separate websites in the last 12 months, ranging from small businesses to adult websites.
See also: MoD computers hit by deadly virus