According to UK newspaper the Daily Express, the hackers, called Team Elite, could also have been able to discover the identity of any user of the website and to find out other websites they had visited previously.
The MI5 holds the identity of agents and informers in terror groups such as Al Qaeda, but security experts at IT security and data protection firm Sophos, said that the hack itself is highly unlikely to have compromised classified information. The security flaw has since been fixed.
Sophos still urged website owners to assess the risks posed by hackers following the breaking of the news of the hacked MI5 website.
"Web infection is one of the primary routes by which hackers are trying to infect innocent computer users - and most of these attacks are occurring via legitimate sites, just like MI5's", said Graham Cluley, senior technology consultant at Sophos. "Website owners should take this as a warning that they are vulnerable to embarrassing hacks if they are not properly protected."
Team Elite has previously hacked into the World Health Organisation’s website and attacked credit card giant Visa’s computer systems, Daily Express wrote.