The independent reviewer of terrorism legislation, David Anderson QC, said that there were now fewer complex plots directed from the tribal areas of Pakistan than there were 10 years ago.
However, he said “al-Qaeda core” still had a role in training and sanctioning individuals in the UK to “self-organise” into groups and carry out attack planning.
He warned that “lone actor” attacks could be more difficult for the security services to detect and disrupt.
“The will and capacity to commit 7/7-style atrocities still exist in the United Kingdom, as demonstrated recently by the Birmingham rucksack bomb plot. However, the bombers’ chances of success have diminished with the marked improvement in MI5’s coverage since 2005,” he said in his annual report.
“Simpler attacks, involving fewer people and less planning, are becoming more common – including against national security targets, as in Northern Ireland – and can be very difficult to detect.”
The threat of such small-scale attacks was underlined by the killing of Fusilier Lee Rigby who was stabbed to death outside Woolwich Barracks in south-east London in May.
Two men have been charged with his murder.
Mr Anderson also highlighted the continued threat from individuals radicalised in “jihadi conflicts” in various part of the world.
“Jihadi conflicts have the potential to radicalise individuals in the UK, and for some individuals who return from fighting abroad, to pose a direct threat to the UK,” he said.
“There is a high risk to UK citizens and interests in the jihadi conflict zones of North, West and East Africa.”
Earlier, he said: “We are certainly not seeing now what we saw in the years around 2005, 2006, which are the large, ambitious, 9/11-style plots perhaps to bring down simultaneously several airliners.
“What we are seeing is a trend towards lone actors and we are also seeing self-organised plots.”
Mr Anderson also defended the controversial European Court of Human Rights, insisting its ruling had not jeopardised security in the UK.
“If you look at what the European Court actually says in terrorism it is good news because it has modified the more rigorous and objectionable aspects of our laws without, I think, decreasing our safety in any way,” he said.