"Leone" emerged from out of the shadows outside my hotel in Suleimaniya, northern Iraq, on a bleak, misty night in January 1998 - just as the crisis between the United States and Iraq over arms inspection was reaching fever pitch.

Local Kurdish officials identified him as a nuclear scientist and when we talked, he seemed well informed.

Leone described himself as an engineer who was a member of the Iraqi Atomic Energy Commission - and a senior official in the secret Iraqi nuclear programme.

To prove his expertise, Leone worked throughout the night drawing detailed diagrams of nuclear weapons in his hotel room.

He said it worked on the principle of the Hiroshima-type bomb, in which high explosives drive pieces of highly enriched uranium together at high velocity. This triggers a nuclear explosion.

Then Leone made another staggering claim - that Iraq had actually conducted a nuclear test before the Gulf War.

Saddam threatened us with the death penalty if we told anybody about it

Leone

It was carried out at 1030 on 19 September 1989 at an underground site 150 kilometres south-west of Baghdad, he said.

"Saddam had threatened us with the death penalty if we told anybody about it."

The location was a militarised zone on the far shore of Lake Rezzaza, which used to be a tourist area. There is a natural tunnel there which leads to a large cavern under the lake.

"We went to a lot of trouble to conceal the test from the eyes of the outside world," said . "The Russians supplied us with a table listing US satellite movements. They were always helping us."

Late last year I turned to the most important defector to reach Europe - Abbas Jannabi, personal assistant to Saddam's son, Uday, for 15 years.

Abbas Jannabi: Appears to confirm the claims

He was imprisoned eight times by his former boss and routinely tortured. He finally fled the country with his family in 1998.

His cousin, Fadhil Jannabi, held a senior position in the Iraqi nuclear programme.

"It's true," he said. "A nuclear test was carried out in 1988 or 1989 in an underground site beside Lake Rezzaza."

Mr Jannabi pointed to the test site on a map of Iraq, and it was close to Leone's location.

"It is a military zone. I doubt whether UN inspectors ever visited it."

He himself had been to the site. He learned of the successful test from Uday, who, he said, was unable to conceal his jubilation.

"They were talking about the test, about their ability to produce a nuclear bomb. They were talking about a new powerful Iraq," said Mr Jannabi. "They had a celebration which was attended by senior officials and ministers."

South Africans 'supplied uranium'

I asked him if it was definitely a nuclear test. He said it was. I asked him who had supplied the uranium for the bomb. "South Africa," he said.

Mr Jannabi claimed that negotiations with the South Africans began in 1986, and the delivery was made in 1988.

So had South Africa really been selling off surplus stocks of highly enriched uranium?

I contacted a former intelligence official under the apartheid regime who had helped procure components for his country's nuclear weapons programme on the black-market.

"The story is true," he said. "About 50 kilograms were sold to the Iraqis. The Americans gave the green light for the deal."

Expert: Bhupendra Jasani

For the final stage of my investigation, I used the latest space technology to check out the Rezzaza test site. If Leone was telling the truth, there might be clues left behind - even though the test apparently happened almost 12 years ago.

I bought pictures of Lake Rezzaza taken in July 1989 - two months before the claimed test - by a French Spot Image satellite and compared them with images from the Indian IRS1D spacecraft, shot in September 2000.

Professor Bhupendra Jasani from King's College, London analysed these images of Lake Rezzaza, and made an important discovery - what looked like the tunnel Leone and Abbas Jannabi had told me about.

The images also showed an army base with some 40 buildings.

Each of the buldings were 40 by 70 metres in size, and a massive missile base was nearby. Sixty per cent of these buildings had been destroyed either in air attacks or by the Iraqis themselves.

"If you wanted to hide something, I guess this is exactly what you would do," said Professor Jasani.

Growing concern

There is growing concern that Saddam's weapons of mass destruction now threaten stability in the region as never before. But has Iraq already tested the atom bomb?

Since I first spoke to Leone, my investigation has come a long way. I have confirmed beyond doubt that UN inspectors missed a very large slice of the Iraqi nuclear weapons programme.

There is also evidence that the Iraqis tested a small nuclear device underground.

Saddam is now a hero in the Middle East because of his stand against Israel, Britain and America.

The tragedy is that the warnings of defectors like Leone have come to nothing. And that Iraq is now emerging as a nuclear power, causing the threat to peace to be far more real than ever before.

Saddam's Bomb: 1815 GMT, Saturday 3rd March on BBC 2.

Reporter: Gwynne Roberts

Deputy Editor: Farah Durrani

Editor: Fiona Murch