Stateless terrorists armed with a nuclear weapon are now a very real threat to the UK, Nick Clegg will warn today.
In a stark message, the Deputy Prime Minister will say materials to make a dirty bomb are so readily available that no police force can hope to contain such a threat.
And while such an atrocity was unthinkable just a generation ago, no country can now afford to ignore the potential risk, he will add.
Last year, The Daily Telegraph disclosed that al-Qaeda is actively trying to secure nuclear material and recruiting rogue scientists to build a radioactive "dirty" bomb.
Mr Clegg’s warning comes as tensions over Iran’s nuclear programme continued to grow yesterday.
David Cameron said an Iranian nuclear weapon was a danger to all and not just the Middle East and did not rule out military action.
Mr Clegg will call for more co-operation between countries to combat the threat of terrorism, crime and economic collapse during a speech in The Hague this evening.
He will say: “And it is only by working together that we have any hope of tackling the new threats.
“Take a terrorist-executed nuclear attack: unthinkable just a generation ago but now a possibility the international community cannot afford to ignore, thanks to an increased availability of nuclear material combined with more information about making the weapons on the internet, as well as thriving smuggling networks.
“That is a stateless threat, impossible for any national police force, no matter how advanced, to contain.
“But together we can agree and enforce the rules that will prevent such attacks. And I’m travelling to a major summit in Seoul later this year to that very end.”
The prospect of a terror group developing a nuclear weapon or dirty bomb has long been a deep concern for the security services.
Leaked diplomatic documents last year showed a leading atomic regulator had privately warned that the world stands on the brink of a "nuclear 9/11".
Security briefings suggested that jihadi groups was also close to producing "workable and efficient" biological and chemical weapons that could kill thousands if unleashed in attacks on the West.
Thousands of classified American cables obtained by the WikiLeaks website and passed to The Daily Telegraph detailed the international struggle to stop the spread of weapons-grade nuclear, chemical and biological material around the globe.
At a Nato meeting in January 2009, security chiefs briefed member states that al-Qaeda was plotting a programme of "dirty radioactive IEDs", makeshift nuclear roadside bombs that could be used against British troops in Afghanistan.
As well as causing a large explosion, a "dirty bomb" attack would contaminate the area for many years.
In a separate development, the Cabinet was yesterday briefed on Iran's nuclear programme amid continuing fears that the Tehran regime is seeking to acquire the bomb.
National Security Adviser Sir Kim Darroch led the presentation, which also featured "a number of experts".
The discussion came after US President Barack Obama held talks with Israeli Prime Minister Binyamin Netanyahu yesterday in Washington to discuss the issue.
The Israelis have shown increasing signs of impatience that diplomatic pressure has so far failed to persuade Tehran to abandon the programme, prompting speculation they could mount air strikes.
But while Mr Cameron insisted an Isareali attack was not the “right thing” at present, he told MPs on the Commons liaison committee: "Nothing is off the table.
"It is difficult to say that because no one wants to see conflict in any way. But I think it's very important that the world sends a message to Iran that a nuclear-armed future is not something that we want to see.
"If the sanctions don't work there will come a moment of a very difficult decision."
He added: ""I don't believe that an Iranian nuclear weapon is just a threat to Israel.
"It is also clearly very dangerous for the region because it would trigger a nuclear arms race but also its a danger more broadly, not least because there are signs that the Iranians want to have some sort of intercontinental missile capability.
"So we have to be clear this is potentially a threat much more widely."