A United Nations report, dated July 18, said there was “no doubt” that a huge shipment of smuggled uranium 238, uncovered by customs officials in Tanzania, was transported from the Lubumbashi mines in the Congo.

Tanzanian customs officials told The Sunday Times it was destined for the Iranian port of Bandar Abbas, and was stopped on October 22 last year during a routine check.

The disclosure will heighten western fears about the extent of Iran’s presumed nuclear weapons programme and the strategic implications of Iran’s continuing support for Hezbollah during the war with Israel.

It has also emerged that terror cells backed by Iran may be prepared to mount attacks against nuclear power plants in Britain. Intelligence circulating in Whitehall suggests that sleeper cells linked to Tehran have been conducting reconnaissance at some nuclear sites in preparation for a possible attack.

The parliamentary intelligence and security committee has reported that Iran represented one of the three biggest security threats to Britain. The UN security council has given Iran until the end of this month to halt its uranium enrichment activities. The UN has threatened sanctions if Tehran fails to do so.

A senior Tanzanian customs official said the illicit uranium shipment was found hidden in a consignment of coltan, a rare mineral used to make chips in mobile telephones. The shipment was destined for smelting in the former Soviet republic of Kazakhstan, delivered via Bandar Abbas, Iran’s biggest port.

“There were several containers due to be shipped and they were all routinely scanned with a Geiger counter,” the official said.

“This one was very radioactive. When we opened the container it was full of drums of coltan. Each drum contains about 50kg of ore. When the first and second rows were removed,the ones after that were found to be drums of uranium.”

In a nuclear reactor, uranium 238 can be used to breed plutonium used in nuclear weapons.

The customs officer, who spoke to The Sunday Times on condition he was not named, added: “The container was put in a secure part of the port and it was later taken away, by the Americans, I think, or at least with their help. We have all been told not to talk to anyone about this.”

The report by the UN investigation team was submitted to the chairman of the UN sanctions committee, Oswaldo de Rivero, at the end of July and will be considered soon by the security council.

It states that Tanzania provided “limited data” on three other shipments of radioactive materials seized in Dar es Salaam over the past 10 years.

The experts said: “In reference to the last shipment from October 2005, the Tanzanian government left no doubt that the uranium was transported from Lubumbashi by road through Zambia to the united republic of Tanzania.”

Lubumbashi is the capital of mineral-rich Katanga province, home of the Shinkolobwe uranium mine that produced material for the two atomic bombs dropped on Hiroshima and Nagasaki in August 1945.

The mine has officially been closed since 1961, before the country’s independence from Belgium, but the UN investigators have told the security council that they found evidence of illegal mining still going on at the site.

In 1999 there were reports that the Congolese authorities had tried to re-open the mine with the help of North Korea. In recent years miners are said to have broken open the lids and extracted ore from the shafts, while police and local authorities turned a blind eye.

In June a parliamentary committee warned that Britain could be attacked by Iranian terrorists if tensions increased.

A source with access to current MI5 assessments said: “There is great concern about Iranian sleeper cells inside this country. The intelligence services are taking this threat very seriously.”

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