Facebook and other social media 'used for cyber-jihad'
Al-Qaeda is using online technology to plan attacks in pursuit of a "cyber-jihad", the government has warned.
The UK's updated counter-terrorism strategy suggests terrorists' use of social media to disseminate information and radicalise people is "commonplace".
It said there was evidence of extremist groups seeking "to invade Facebook".
Home Secretary Theresa May said the UK must react to this threat but stressed Osama bin Laden's death had left al-Qaeda in its weakest state since 2001.
Although the international terror threat level to the UK was lowered on Monday from "severe" to "substantial", it means there is still a "strong possibility" of an attack.
Last year the government identified cyber warfare as one of the most serious "Tier 1" threats to the UK's security and set aside £650m in additional money to make key infrastructure more resilient.
Defence Secretary Liam Fox recently revealed that his department had dealt with more than 1,000 "potentially serious" attempted cyber attacks in the past year.
The government's updated counter-terrorism strategy, published on Tuesday, warns that the number of attacks on IT systems will likely increase and that extremists were increasingly sophisticated in their use of social networking and video sharing sites.
"Since the death of Osama Bin Laden, al-Qaeda has explicitly called not only for acts of lone or individual terrorism but also for 'cyber-jihad'," the document said.
"There have been a number of attempts by terrorist and extremist groups to 'invade' Facebook.
"Twitter will be used to re-post media or forum articles enabling extremist content to be shared more quickly, widely and amongst people who would not normally search for extremist content."
Advances in technology - such as software to encrypt mobile phone calls and texts, file sharing networks and cloud computing - meant terrorists were able to store and share material online much more easily while also disguising their actions.
Mrs May said the UK must learn the lessons from how past terrorist incidents - such as the 7/7 bombings in London and the 2008 Mumbai shootings - had been planned and "work harder" to tackle radicalisation via the internet.
"Our response must improve to keep pace," she said.
"Terrorists are increasingly using online technology, including Google Earth and Street View, for attack planning. While radicalisation continues primarily to be a social process, terrorists are making more and more use of new technologies to communicate their propaganda."
Bin Laden impact
Despite the number of attacks which have reportedly been foiled in recent years, there continues to be criticism of the small number of successful prosecutions of terrorist suspects.
The home secretary said police were now able to question suspects further after they had been charged in the event of "substantial" new evidence coming to light.
This, she said, should enable the authorities to build "more robust" cases and explore new lines of inquiry as part of an ongoing investigation.
More broadly, the home secretary said al-Qaeda's ability to co-ordinate international attacks had been "weakening" for many years and the killing of Osama Bin Laden by US security forces in May had further disrupted its activities.
"The leadership group of al-Qaeda, based primarily in Pakistan, is now weaker than any time since 9/11," she said.
"Bin Laden was a more central and more pivotal leader than we knew. A gap has been left which might never be filled. Although it has proved resilient in the past, al-Qaeda as a centralised command and control organisation may not survive the fall of Bin Laden and the rise of democracy in the Arab and Muslim majority world."
However, Mrs May said the threat from Northern Ireland-related terrorism had increased and people must remain vigilant.