The truck, owned by Kent-based shipping firm Orient Transport Services, was carrying 10 soil-testing devices containing radioactive caesium 137 and americium-beryllium, Britain's Daily Mail reported at the weekend.

The paper said it was stopped at a checkpoint on Bulgaria's northern border with Romania after a scanner detected abnormal radiation levels.

The Mail quoted a Bulgarian customs official as saying that although documentation listed the shipment as destined for the Ministry of Transport in Tehran, the lead-lined boxes containing the devices were addressed to Iran's Ministry of Defense.

Andrew Maclean, a director of Orient Transport Services told the paper the shipment was perfectly legal. "We had a letter from the (British Department of Trade and Industry) confirming that no export license was needed to send these items to the Iranians," he said.

"We also alerted customs officials about the goods we were transporting before they left the U.K. and the truck carried all the appropriate warning symbols to alert officials and the emergency services of what it was carrying."

The paper said a spokesman for the department confirmed that "This sort of material ... is not covered by our export controls."