As the US special forces Hercules flew low over the desert in March 2003 before landing behind enemy lines, Hazim al-Shalan prepared to step onto Iraqi soil for the first time in almost 20 years.

Shalan — the exiled leader of a Shi’ite tribe with 1m members worldwide and 250,000 in southern Iraq — had already been helping British and American intelligence for several months. Now his task was to organise support among the Shi’ite population for the imminent invasion.

After the fall of Saddam Hussein he rose from resistance organiser to provincial governor and defence minister. Now, however, he is back in London saying that he is wholly disillusioned.

Lucky to be alive after 15 attempts on his life and accused of fraud by his former colleagues, Shalan warned last week that the country is gradually being taken over by Iran with devastating potential consequences.

Talking exclusively to The Sunday Times, he said that the Iranians influence the Iraqi police and army and even the interim government.

More than 460 Iranian intelligence agents have been captured in the country, but many thousands more are openly operating, he said.

According to Shalan, the Iranian intelligence service began infiltrating Iraq two months before the allied invasion.

When Saddam began withdrawing troops from outlying regions to protect Baghdad, Iranian intelligence officers “entered through the desert, between 200 and 250 of them carrying just Kalashnikovs and light communication equipment”.

After the invasion, the infiltrators reported back to Tehran that American troops were busy providing critical services such as water, food and medical assistance to Iraqis.

“The Iranians decided there was an opportunity to send large numbers of people very quickly into Iraq. Thousands of Iranians and Iraqi exiles who had joined the militia in Iran began arriving with money to buy houses,” said Shalan.

They sought positions of influence on the new councils and other bodies being formed.

“I talked to the Americans about it, told them trouble was building,” said Shalan, who was at the time a governor in charge of the city of Diwaniyah, with 1m residents. “But the Americans said the Iranians were not effective and not a threat.

“They had received assurances from Tehran that Iran was not trying to get involved.”

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