LONDON: Islamist militants are less capable of organising plots on the scale of 9/11 but small groups of jihadists returning from Syria pose a grave threat to British security, the country's two most senior counter-terrorism officers said on Thursday.

Every year, Britain's security services are faced with at least one plan to carry out an attack on the level of the July 7, 2005 suicide bombings on London, known as the 7/7 attacks, which killed 52 commuters, the officers said.

While the al Qaeda leadership in Pakistan and Afghanistan can no longer coordinate such attacks on the level it once could, the threat is now more complex with more autonomous militants just as dangerous.

"There is no doubt the big sophisticated 9/11 type plot, 7/7 type plots, are much harder to organise," said Stuart Osborne, Britain's Senior National Coordinator for Counter Terrorism.

"They did need a lot of overseas direction and I think it would be fair to say that some of the al Qaeda leadership have sort of said that's good if you can do it, but if not, any attack, whatever you can, at whatever size is useful."

He said small groups of Islamist militants, many who had travelled for training and to fight in places such as Syria, Yemen and Somalia, were forming at short notice and without the broader command and control structures that previously existed.

"The innovation that they are now creating has actually potentially made it just as dangerous, if not more so," he Osborne told reporters.

Britain's national threat level from international terrorism is currently assessed at "substantial" - the third-highest level of five, meaning an attack is a strong possibility.

That is one notch lower than it has been for most of the years following the 7/7 bombings, and there have been no deadly attacks by militants on the British mainland since then.

The government and security services say a serious threat remains, and last month three British Islamists were found guilty of plotting a campaign of bombings to create what one described as "another 9/11".

"The threat has not gone away but is changing in its nature. We can't be and we absolutely are not complacent in the face of those challenges," said Cressida Dick, head of specialist operations for London police and the most senior counter-terrorism officer in the country.

Foreign Secretary William Hague warned last month the ongoing conflict in Syria would breed a new generation of battle-hardened militants who would pose a risk to Britain and other European countries.

"The opportunities to exploit vulnerabilities in Syria and similar places are now going to be looked at. They will go where ever they feel they can get their training at the easiest possible opportunity," said Osborne.

"Some who have been trained actually are becoming quite self-motivated, they're beginning to plan in small groups which are hard to detect."

While Osborne and Dick declined to say how many Britons had gone to Syria, saying it was a small number, Western security sources say dozens of impressionable young men have joined the conflict there from Western European countries.