Britain and the Olympics face a growing threat from "lone wolf" terrorists who have taught themselves how to build bombs using al Qaeda publications, a leading security think tank has warned.
Amid fears of budget cuts to the intelligence services, the Royal United Services Institute (Rusi), which employs former spies as senior analysis and has close connections to the British Armed Forces, said in a study that Africa and the Yemen were also rapidly becoming major training areas for terrorists.
Rusi's latest UK terrorism analysis said: "The growing number of lone wolves who are radicalised and then indulge in some 'terrorist tourism' are not normally part of a well-organised pipeline of guerrilla fighters going to the jihad.
"More usually, they are personally connected to family or friends who have already gone to a conflict zone.
"Britons are thought to make up about 25% of the 200 or so foreign fighters that the al Shabaab group in Somalia currently fields, and who are engaging in a deepening war on neighbouring Kenya and its tourist trade."
Nigeria is a newly identified potential breeding ground for radicalised Islamists with strong connections to immigrants in the United Kingdom, Rusi added, following a year of violence in Nigeria blamed on Boko Haram.
"It would be surprising if both the tactics and the tensions underlying Boko Haram's campaign do not spread some ripples among the Nigerian ethnic communities in the UK," Rusi's experts said.
Spending on the intelligence services - GCHQ, MI5 and MI6 - has shot up 250% over the last 10 years since 9/11 to £3.5bn.
From 2010, they faced a cut of around 11.3% - which has largely been put off as a result of heavy spending on security for the Olympic Games.
The institute identified four main threats to the sporting spectacular.
First, a "traditional" attack organised by al Qaeda's "core" - which has been badly eroded by American drone strikes in Pakistan and the Yemen and by the killing of Osama bin Laden last year.
Second, a hitherto unexamined problem with the release of dozens of men convicted of terrorist offences from British prisons, who are expected to finish their sentences over the next 12 months.
Third, it said "the lone wolf ...is perhaps the bigger threat and could extend to non-Jihadist terrorism, as we saw in the case of the Norwegian bomber Anders Behring Breivik".
"However this type of action is more prevalent amongst Jihadists...The terrorist threats to the UK are becoming increasingly diverse," the report added.
"The plots could be less elaborate and well planned than those involving al Qaeda core, and thus less predictable and more volatile.
"Even this threat is set to evolve in a significant way, however, as more experienced lone wolf terrorists are likely to be returning to Britain in the next couple of years, not from training camps in Pakistan and via airports in Karachi and Dubai; but from wars in Somalia, Yemen or Nigeria.
"Also from the renewed violence in Iraq, and from destinations and via routes that will be far more difficult for security services to monitor."
Lastly, Rusi said there remained a dissident Irish republican rump which might try to disrupt the Olympics - although most experts see this as a very distant prospect.
Rusi, perhaps with a nudge from its sources in the intelligence community, warned that future cuts to the spooks' budgets should be carefully scrutinised.
"In many respects, the Olympic Games are likely to prove a major turning point in the allocation of resources for UK counter-terrorism," it said.
"If the Olympics end with no major incident, this will mean that no terrorist attack has taken place since the 2007 attack on Glasgow International Airport.
"Therefore it is difficult to see the appetite for counterterrorism spending and staffing levels to continue at the rates sustained throughout the 2000s.
"Clearly the lack of a successful attack in five years is in no small part due to the large investments and changes that have been made in the security mechanisms for countering such threat. Yet, there is a sense of inevitability that change is on its way."