A secret police memo warns: "This material may become radioactive if ignited ... vehicle must not be approached by police."

All police forces in the UK, including the eight in Scotland, have been alerted to keep a lookout for the van, a T-reg Citroen Berlingo.

The intelligence service MI5 and defence chiefs have been alerted.

Nuclear experts last night warned the missing material could be used by terrorists in an attack on cities.

Police yesterday admitted the van and the uranium had gone missing but refused to give any more details.

The radioactive waste is carried for miles in the atmosphere if detonated with a conventional explosive. Small particles of depleted uranium dust lodge in victims' lungs and it has been blamed for causing cancer.

The van was stolen from an industrial estate last Monday.

The theft happened in the Purfleet area of Essex before police raised the alarm around the country.

The substance - which is used to make shells and other ammunition - has been blamed for causing illness among hundreds of Gulf War veterans and civilians in Iraq.

Last night, depleted uranium expert Professor Malcolm Hooper said: "Terrorists would want to obtain this. Our government and the United States have warned it could be used in a terror attack."

Professor Hooper, of Sunderland University, added: "This is very serious and the authorities will be keen to track down the depleted uranium and identify the people who have it.

"It's difficult to say how much damage 15 kilos of depleted uranium would cause.

"But it could certainly be converted to make an effective `dirty bomb'.

"A lot of depleted uranium is stored in the United Kingdom but it has to be securely protected."

A "dirty bomb" is easy to construct if terrorists have access to any form of uranium.

The weapon would have a devastating effect and cause widespread panic.

The nuclear waste product is the prize of terror fanatics such as Osama bin Laden's al-Qaeda organisation.

A contaminated area could be uninhabitable for years and victims exposed to radiation would have to be monitored for the rest of their lives.

There have been around 180 cases of terrorists trying to smuggle radioactive material since 1993, most of it depleted uranium, known as U-238.

The material is widely used by the military and is used as balance weights in heavy aircraft.

However, the growing concern about it falling into the hands of terrorists has prompted a huge security crackdown to ensure it is held safely.

DU is a waste product of the process that produces enriched uranium for use in atomic weapons and nuclear power plants.

Much like natural uranium, it is both toxic and radioactive.

Half a million tons of DU exists in the US and must be safely stored or disposed of by the Department of Energy. It has a half-life of 4.5billion years.

Yesterday, Dr David Sanderson, of the Scottish Universities Environmental Research Centre, said: "I'm astonished that depleted uranium was in the back of a van in the first place.

"It can be a very harmful substance, depending on what form it's in.

"Shavings of depleted uranium or the substance in its powder form is very dangerous.

"Even if the quantity missing is in solid metal form, it's very easy to process it to powder."

The Government gives DU to arms manufacturers.

Because it is extremely dense, when turned into a metal, it can can be used to make a shell that penetrates steel.

In addition, when it strikes steel, heat from the friction causes it to burn.

When DU burns, it spews tiny particles of poisonous and radioactive uranium oxide in aerosol form, which then travels for miles in the air.

Humans can inhale the small particles, which lodge in vital organs.

DU is also considered at least a contributing cause in more than 130,000 reported cases of Gulf War Syndrome.

The chronic symptoms include increases in cancers to memory loss, acute pain, fatigue and birth defects in veterans' children.

Essex police said they believe the van was stolen by an opportunist thief. But a spokeswoman admitted the van and its alarming contents have yet to be traced.

She said: "A van containing a small amount of depleted uranium was stolen. Inquiries are continuing." The movement of uranium is what sparked the latest row over the war in Iraq after the government claimed Saddam Hussein was trying to buy it from Niger.

The claim, in a controversial document outlining the justification for attacking Iraq, was that Saddam wanted the uranium for his weapons of mass destruction programme.

But yesterday, it emerged the CIA had backtracked after using the same claim, saying the evidence came from a dubious source.

Yesterday, Foreign Secretary Jack Straw admitted the CIA raised concerns about the claim in the Iraqi dossier.