Earlier this month Boris Johnson’s government published a complete review of its defence and foreign policy.
The headlines were taken by the decision to reduce the British Army to 70,000 soldiers - the smallest it has ever been - to increase nuclear warheads and to task a Royal Navy ship with the protection of undersea cables.
But buried in the report was this sentence: “It is likely that a terrorist group will launch a successful CBRN (chemical, biological, radiological and nuclear) attack by 2030.”
David Lowe, a retired police officer and senior research fellow at Leeds Beckett University Law School, said: "It is scary. The threat is quite remote but you can never say never."
He told Sputnik: "If you are going to do a dirty bomb you need a high level of sophistication, not just to make one but to store it. It’s not like one of the knife or vehicle attacks we’ve seen recently. You need good academic knowledge of how to build it and use it."
Mr Lowe said MI5 and the Metropolitan Police’s counter-terrorism officers keep a very close eye on potential terrorists and he said it would need an epic failure, of the kind which failed to spot the efforts being made by al-Qaeda in the run-up to the 9/11 attack, for the security forces to miss a dirty bomb plot in Britain.
He said: “The big difference is that there were warnings before 9/11 which were ignored. We don’t ignore warnings now.”
The review said: “The main sources of terrorist threat are from Islamist and Northern Ireland-related terrorism and far-right, far-left, anarchist and single-issue terrorism.”
Al-Qaeda and ISIS do not pose the same threat they did in the past and most Islamist attacks in recent years have been by “lone wolves” using low-tech weapons like knives, machetes or trucks.
Mr Lowe said he doubted they possessed the financial or logistical capability to produce a chemical or biological weapon of mass destruction.
He said such a device could be hidden in something as small as a briefcase or rucksack but he said: “The smaller it is, the less potent.”
Mr Lowe said the most likely means of delivering a dirty bomb in a city like London was hidden inside a van.
While Islamists remain the most likely to use a dirty bomb, there is always the possibility of an unforeseen threat.
In 1995 a Japanese doomsday cult called Aum Shinrikyo released sarin gas in the Tokyo subway, killing 13 people.
Mr Lowe said it had taken “everyone by surprise.”
The cult leader, Shoko Asahara, reportedly hoped the attack would trigger an apocalypse. He was sentenced to death in 2004 and hanged in 2018.