A Thai national has been sentenced to 40 years' imprisonment after pleading guilty to charges of exporting rhino horns from South Africa, in the country's strongest illegal wildlife sentence to date.

Chumlong Lemtongthai admitted to playing a large part in a scheme that used white rhino trophy hunts in South Africa as a cover for smuggling horns to black markets in Asia. He employed Asian nationals to pose as hunters and take part in organised hunts on game farms in the North West province.

Charges against three South Africans and two others Asian nationals, the co-accused, were however dropped without explanation.

Lemtongthai told Johnannesburg's Kempton Park magistrates court: "I humbly apologise to the court and to the people of South Africa for my role in this matter. I appreciate that the emotions of all animal lovers in South Africa are running very high and that I was part of the problem."

Jo Shaw, rhino co-ordinator for WWF-SA, said: "These higher-level arrests and convictions are critical to disrupting the illegal trade chains used to move rhino horns into illicit markets in Asia."

Areas of Asia, in particular Vietnam, have increased their demand for white rhino horn powder in recent years. Wrongly believed to enhance sexual performance, cure hangovers and even cancer, a growing wealthy class in Asian society have begun to pay more than ever for rhino horns. The result has been a rapid rise in rhino horn poaching in South Africa, culminating in a record rise in 2012, with more than 450 rhinos killed in the country this year already.

Around half of the poaching occurs in the Kruger national park, in the country's north-west.