A controversial online auction of rhino horn set to open Monday has been delayed by two days after a legal challenge delayed the South African permit for the sale, the organisers announced.

Outraged conservationists say the three-day auction could undermine the global ban on rhino products, and the South African authorities had moved to ban the sale.

But in an 11th-hour decision on Sunday, the High Court in Pretoria ruled in favour of the auction's South African organiser, John Hume, who runs the world's biggest rhino farm.

His lawyer had argued that the seller's permit had been approved but not issued by the authorities in South Africa, where a ban on domestic rhino horn trade was lifted three months ago.

The permit was handed over on Monday morning, and the homepage of the website for the sale -- which is in English but offers versions in Chinese and Vietnamese -- was changed to say that bidding would start on Wednesday at 2:00 pm (1200 GMT).

"It was supposed to start today, but we have only received the permit this morning," said one of the auction officials, who did not have authority to speak to the media. 

The environmental affairs ministry said the permit had originally been withheld because it had been issued by a junior official, instead of the minister.

And in a statement Monday, the ministry said Hume could sell the horns only to people who have obtained buyers' permits, none of which had been issued as of yet.

The ruling in April to lift the eight-year moratorium on rhino horn trade was said to have little impact outside South Africa because a ban on international trade is still in force.

But rhino breeders say they believe open trade is the only way to stop poachers from slaughtering the animals.

Hume, who owns 1,500 rhino on his farm north of Johannesburg, has stockpiled six tonnes of horns and wants to place 264 horns, or about 500 kilogrammes' (1,100 pounds), under the hammer.

He harvests the horns by tranquilising the animals and dehorning them, a technique used to ward off poachers.

South Africa is home to around 20,000 rhinos, or about 80 percent of the worldwide population, but in recent years the country has suffered record slaughters by poachers.

Rhino horns are highly prized in Asia, and are estimated to fetch up to $60,000 a kilo on the black market, exceeding the price of gold or cocaine.

They are composed mainly of keratin, the same component as in human nails, and are sold in powdered form as a supposed cure for cancer and other diseases -- as well as a purported aphrodisiac -- in Vietnam and China.

South Africa has over 300 private rhino breeders who say they have spent more than two billion rand ($150 million) to protect their herds over the past nine years.

Hume and some other campaigners say poaching can be halted only by meeting the huge demand from Asia through legally "harvesting" horn from anaesthetised live rhinos.

But animal rights activists charge that the legal sale of rhino horns will only fuel poaching.

Johan Van Eyk of Van's Auctioneers, who will conduct the auction, said no opening price for bids had been set, as this will be the first auction of rhino horns ever.