A major counter-terrorism investigation has been launched after several kilograms of uranium was seized at Heathrow airport.

The deadly nuclear material - which could potentially be used in a ‘dirty bomb’ - arrived on a flight from Oman, in the Middle East, on December 29.

The shipment was addressed to an Iranian-linked firm in the UK, it is understood.

Sources said the uranium was ‘not weapons-grade’ - and so could not be used to manufacture a thermo-nuclear weapon.

But the security services are understood to be investigating whether the undeclared package could have been destined for an improvised nuclear device, known as a ‘dirty bomb’.

Such a device - which has long been a nightmare scenario for counter-terror experts - combines conventional explosives with nuclear material to disperse a lethal radioactive plume.

The package originated in Pakistan before arriving at Heathrow’s Terminal Four aboard an Oman Air passenger jet from Muscat.

A shipment of uranium has been seized at Heathrow airport

The undeclared material was discovered on December 29 on a passenger flight which arrived from Oman

Police have not made any arrests.

A source told the Mail: ‘The package contained kilos of uranium - but it was not weapons-grade.'

Separately, a source told The Sun there is an overwhelming 'concern over what the Iranians living here wanted with non-disclosed nuclear material'.

An unnamed source told the publication: 'The race is on to trace everyone involved with this rogue non-manifested package.

'Security bosses are treating this with the seriousness it deserves. Protocol was not followed and this is now an anti-terror operation.'

Specialist scanners picked up on the undeclared parcel as it was transported to a freight shed. 

Specialist scanners picked up on the undeclared parcel as it was transported to a freight shed

Border Force agents isolated the shipment in a radioactive room and, upon determining it was uranium, called in counter-terror police.

Met Police told MailOnline: 'We can confirm officers from the Met's Counter Terrorism Command were contacted by Border Force colleagues at Heathrow after a very small amount of contaminated material was identified after routine screening within a package incoming to the UK on 29 December 2022.'

Commander Richard Smith said: 'I want to reassure the public that the amount of contaminated material was extremely small and has been assessed by experts as posing no threat to the public. 

'Although our investigation remains ongoing, from our inquiries so far, it does not appear to be linked to any direct threat. 

'As the public would expect, however, we will continue to follow up on all available lines of enquiry to ensure this is definitely the case.

'However, it does highlight the excellent capability we and our partners have in place to monitor our ports and borders in order to keep the public safe from any potential threats to their safety and security that might be coming into the UK.'

'No arrests have been made at this time and officers continue to work with partner agencies to fully investigate this matter and ensure there is no risk to the public.

'The material has been identified as being contaminated with uranium.'

Specialist scanners picked up on the undeclared parcel as it was transported to a freight shed. Pictured: A nuclear storage facility

A Home Office spokesman said: ‘We do not comment on live investigations.’

Hamish De Bretton-Gordon, former commander of the UK’s nuclear defence regiment, said: ‘Uranium can give off very high levels of poisonous radiation. It could be used in a dirty bomb.

‘The good news is the system worked and it has been interdicted.’

Forensic teams are understood to still be examining the nuclear material.

As long ago as 2003 the then head of MI5 warned that it was ‘only a matter of time’ before a dirty bomb or chemical weapons attack was launched on a major Western city.

Eliza Manningham-Buller said intelligence reports suggested ‘renegade scientists’ had given terrorist groups the information they needed to create such weapons.

‘My conclusion, based on the intelligence we have received, is that we are faced with a realistic possibility of some form of unconventional attack that could include chemical, biological, radiological or nuclear attack,’ she said.

‘Sadly, given the widespread proliferation of the technical knowledge to construct these weapons, it will be only a matter of time before a crude version of a CBRN is launched on a major western city.’

In 2004 British security services arrested Dhiren Barot, a Muslim convert who planned to assemble and use dirty bombs in the UK and the US to kill members of the public.

He was jailed for 30 years.

Sources said the uranium was ‘not weapons-grade’ - and so could not be used to manufacture a thermo-nuclear weapon

The Home Office-backed ‘ProtectUK’ website, which offers advice on terror threats, currently says: ‘A UK attack plot using a radiological weapon is highly unlikely because there are significant challenges in acquiring suitable radioactive sources, which are subject to controls.’

Last year, Former Washington official Robert Joseph told MailOnline Iran is a nuclear weapons state with enough uranium to build 'one, if not two' bombs. 

He said: 'The International Atomic Energy Agency (IAEA) has documented that Iran has 60% of enriched uranium, enough for at least one if not two bombs.

'We have been saying for years 'they're approaching this breakout point and we've really got to negotiate with them.' They're there.'

Joseph was the chief negotiator to Libya in 2003 and is credited with convincing Colonel Muammar Gaddafi to give up his nuclear weapons programme. 

MP Matthew Offord said at the time Iran were 'regularly testing ballistic missiles, and they are seeking to get enough uranium that they are able to produce a weapon'.