The ability of Britain's security institutions to fight the threat from online activists was cast into doubt after hackers penetrated Scotland Yard's anti-terrorism hotline and leaked conversations between staff online.
The security breach came as another hacking group, which attacked a host of Government websites last week, vowed to resume its offensive this weekend with attacks on GCHQ and the Ministry of Justice.
Scotland Yard said it was investigating the attack after a teenage activist listened in on a conversation between anti-terrorist officials and posted the results online. The group also "phone bombed" the hotline with 700 calls, preventing anyone else from calling in. The group, an international team of eight hackers known as "TeaMp0isoN", who claim never to have met, told i that it was easy to break into the "very old" phone system to record the conversation, in which an official briefed a colleague about the "phone-bomb" attack.
Dr Richard Overill, a senior lecturer in computer science at King's College London, said: "It is appalling that this hack could be carried out in the way they say it was." The attack raises questions about national security and the level of telecommunications security at Scotland Yard after hackers from another group, Anonymous, broke into a conference call in February instigated by the FBI and published the discussion online.
Hackers from rival groups said there would be no let-up in attacks on what they saw as the establishment. Members of Anonymous told i they intended to wage a sustained campaign in protest at decisions to extradite British citizens to America and at Government plans to allow police and security services greater powers to trawl private email accounts.
One of the founders of the group, known only as TriCk, posted a conversation between a hacker and a Scotland Yard terror official. "TriCk" told i that the security breach was in retribution for Britain's stance on Babar Ahmad and other terror suspects who lost their fight against extradition at the European Court of Human Rights this week. The group, which emerged in 2009, has attacked the websites of, or exposed personal information about, Nasa, the former Prime Minister Tony Blair, BlackBerry and the United Nations.
A spokesman for the Ministry of Justice said: "We take the security of our website extremely seriously and we are working with our suppliers to minimise the risk to the site from future attacks."
A spokesman for GCHQ did not respond to a request for comment.