Armed with a hidden camera, Ms Chen and a colleague from the EIA risked their lives posing as buyers.

A tip-off from a former dealer and a hastily drawn map led the investigators from Taiwan to China, where the undercover work began.

Ms Chen and a Taiwanese friend posed as brother and sister while two other EIA investigators present for extra protection initially kept out of the picture so as not to arouse suspicion.

Their first break came when a middleman was found through an office said to sell rhino and tiger products, and bear gall bladder.

When the investigators approached the office, they were told the products were no longer available because of a government ban. But when Ms Chen telephoned the man privately, he said he could get rhino horn for them from another source.

''I told him we wanted to see the stock, and he said that could be arranged. But my partner had to go back to Taiwan,'' Ms Chan said.

''It was too dangerous to see the dealer on my own but we could not introduce a Westerner without arousing suspicions.

''In the end, we got my friend to call while the middleman was with me, and pretend to be my boss.

''He told the middleman he was going to send his partner, a Westerner named Steve. I told the middleman Steve and I were married and that Steve was a South African who had been involved in the rhino trade there.'' The three of them went to Wuchuan to meet the dealer and see the stock.

''[The warehouse] was dark, there was no electricity. There were about 20 big sacks of rhino horn, and we said we wanted to check the quality.

''We said we had a special machine to test the horn's quality when, actually, we were setting off a flash to light up the horn.

''Then we asked if we could look at one in the daylight, and asked if we could take a photograph, so I filmed while Steve took pictures.'' But when they reviewed the film, they were disappointed with the quality. They volunteered to go again.

Ms Chen admits she was initially reluctant to return. ''I just wanted to get out but Steve thought we should go back again. We did not want to leave with any regrets. It gave us a chance to get better footage and find out how the horns were smuggled out.'' The investigators told the dealer they were coming to choose their stock.

They even persuaded the dealer to line up the horns outside - providing shocking footage for the hidden camera.

''[The dealer] said he could use a military police escort inside China. He could deliver to Hong Kong but said he would have to charge more for insurance. I think he meant for the smugglers,'' said Ms Chen.

The trusting dealer even invited them to stay the night in Wuchuan but they declined.

They flew out of China the next morning. From the safety of Taiwan, they told the dealer the crackdown on rhino horn smuggling by the Taiwanese Government had sent the market plummeting, and they no longer wanted to buy.

Evidence of the illegal dealer's activities was given to the Chinese embassy in London, and EIA representatives understand the business was raided.

Hong Kong Government representatives shown the film last week have promised to review measures for tackling wildlife smuggling in the territory.

The video, shot this July, shocked the world when it was aired at the United Nations Convention on International Trade in Endangered Species (CITES) and on BBC and CNN.

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