Terrorism plot size of 7/7 attacks 'foiled every year'
Police are foiling a terrorism plot as big as the 7 July attacks every year, a senior officer has said.
Deputy Assistant Commissioner Stuart Osborne added that the threats against the UK were constantly changing.
Mr Osborne, senior national co-ordinator for counter-terrorism, said Islamic extremists were planning in smaller groups to avoid detection.
It came as the Home Office revealed the number of terror arrests had risen by 60% in the year to September 2012.
Four suicide bombers killed 52 people and injured more than 700 on London's public transport system on 7 July 2005.
The co-ordinated attack was linked to al-Qaeda.
Mr Osborne said: "On average we've probably had about one potential attack planned with an intent to create something similar to July 7 every year.
"The UK threat as we stand today remains at substantial, which means that a terrorist attack remains a strong possibility and could occur without warning."
Mr Osborne said the hatching of plans in small groups followed the foiling of large-scale terrorism plots.
These included a scheme to blow up passenger jets with bombs disguised as soft drinks in 2006, and more recently a Birmingham-based plot to set off up to eight rucksack bombs.
"Some who have been trained actually are becoming quite self-motivated, they are beginning to plan in small groups which are hard to detect," he said.
"There is no doubt that the big sophisticated 9/11 or 7/7 type plots are much harder to organise, they did need a lot of overseas direction and some of the al-Qaeda leadership have said that's good if you can do it, but if not, any attack whatever you can do at whatever size is useful.
"We are seeing more small groups getting together at shorter notice and more people wanting to do things without that broader command and control."
Mr Osborne said al-Qaeda remained the greatest terror threat to Britain.
The most recent trend was for would-be jihadists to travel to Syria for training and to get involved in fighting, he added.
Mr Osborne said there was also a worry that terrorists would learn "criminal tradecraft" within the UK, making their actions harder to uncover.
Earlier, the Home Office said a total of 245 people had been held on suspicion of terrorism-related offences over the 12 months to September 2012, compared with 153 the previous year.
Of those arrested, 45 (18%) were charged with a terror-related offence, with 10 convicted and 25 awaiting trial. One of the remaining 10 had been acquitted, while the other nine had been convicted over non-terror related offences.
There were 134 prisoners classified as terrorists or domestic extremists at the end of September last year.
A total of 2,291 terrorism arrests had been made since the September 11 attacks on America in 2001.
The figures also showed that special police powers to stop and search people for terrorist material had not been used once since they were introduced in March 2011.
The figures, provided to the Home Office by the Association of Chief Police Officers, revealed the number of people arrested by police in England, Wales and Scotland, where it was suspected they were connected to terrorism.