The disappearance and likely defection of a senior figure in the Iranian regime could provide the West with an intelligence bonanza and deal a blow to the credibility of President Mahmoud Ahmadinejad.

Ali Reza Asgari, a former Iranian defence minister, disappeared in Istanbul last month and is now cooperating with Western intelligence agencies.

Although he left government two years ago, his involvement in building up the Hezbollah movement in Lebanon in the 1980s and 1990s could shed light on one of the murkiest aspects of Iran’s foreign operations.

During this period Hezbollah was transformed from a small Shia Muslim militia into Lebanon’s dominant para-military force responsible for fighting the Israelis, running a parallel government in southern Lebanon and launching attacks against Western targets. Its accomplishments are credited with the training, finance and arms provided by the Iranian Revolutionary Guards, where Asgari served for many years.

“He is a significant figure,” said one Western source, who follows Iran closely. “It has so far been very difficult to get reliable information on how Iran ran its operations in Lebanon. This could be a big break.”

The Israelis, in particular, will be eager to gain access to Asgari’s intelligence, after the failure of their military operation last summer to dislodge Hezbollah fighters from southern Lebanon.

Dr Ali Ansari, a expert on Iran at St Andrew’s University, said that Asgari may also become a damaging public relations problem for Tehran.

“There has never been a defection from Iran in the 27 years since the revolution,” he told The Times.

“If it emerges that a senior figure has defected and is prepared to go public with criticism of Ahmadinejad that could be hugely embarrassing for the Iranian leader.”

Asgari is thought to have intimate working knowledge of Iran’s defence industry and access to some of the country’s most closely guarded secrets, although his name has not been connected to the nuclear programme.

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