Anti-terrorist officers and MI6 are now investigating a wider British-based plot allegedly to supply Iran with material for use in a nuclear weapons programme. One person has already been charged with attempting to proliferate 'weapons of mass destruction'.

During the 20-month investigation, which also involved MI5 and Customs and Excise, a group of Britons was tracked as they obtained weapons-grade uranium from the black market in Russia. Investigators believe it was intended for export to Sudan and on to Iran.

A number of Britons, who are understood to have links with Islamic terrorists abroad, remain under surveillance. Investigators believe they have uncovered the first proof that al-Qaeda supporters have been actively engaged in developing an atomic capability. The British company, whose identity is known to The Observer but cannot be disclosed for legal reasons, has been wound up.

A Customs and Excise spokesman said: 'We continue to investigate allegations related to the supply of components for nuclear programmes including related activities of British nationals.'

It is not clear whether all of those involved in the alleged nuclear conspiracy were aware of the uranium's ultimate destination or of any intended use.

British agents believe Russian black-market uranium was destined for Sudan, described as a 'trans-shipment' point. The alleged plot, however, was disrupted in early 2006, before the nuclear material reached its final destination.

Roger Berry, chairman of Parliament's Quadripartite Committee, which monitors arms exports, said: 'With the collapse of the Soviet Union there was always the question over not just uranium but where other WMD components were going and how this could be controlled. Real credit must go to the enforcement authorities that they have disrupted this. The really worrying aspect is that if one company is involved, are there others out there?'

Politically, the allegations hold potentially huge ramifications for diplomatic relations between the West and Tehran. Already, tensions are running high between Iran, the US and the European Union over the true extent of Iran's nuclear ambitions. Iran refuses to suspend its nuclear programme in the face of mounting pressure, arguing its intent is entirely peaceful and solely aimed at producing power for civilian use.

Investigators are understood to have evidence that Iran was to receive the uranium to help develop a nuclear weapons capability. 'They may argue that the material is for civilian use but it does seem an extremely odd way to procure uranium,' said Berry.

Alleged evidence of Sudan's role will concern British security services. The East African state has long been suspected of offering a haven for Islamist terrorists and has been accused of harbouring figures including Osama bin Laden who, during the mid-Nineties, set up a number of al-Qaeda training camps in the country.

Details of the plot arrive against a backdrop of increasing co-operation between Sudan and Iran on defence issues, although the level of involvement, if any, of the governments in Khartoum and Tehran in the alleged nuclear plot is unclear.

However, circumstantial evidence suggesting that elements within both countries might be colluding on military matters has been mounting in recent months. A Sudanese delegation visited Iran's uranium conversion facility in February, while the East African country reportedly recently signed a mutual defence co-operation pact with Iran, allowing Tehran to deploy ballistic missiles in Sudan.