The head of the Customs' marine enforcement division, Kwok Yim-kwan, said: 'We discovered an empty fuel tank which is connected to the engine room. We suspected it may have been used as a secret compartment to hide smuggled goods, but we were unable to find its entrance.'

After an hour-long search, the Customs officers discovered three wires concealed above the engine room.

When the wires were joined, a false floor about a metre square lowered into the secret compartment where the ivory was packed.

A 37-year-old Hong Kong master and 27-year-old mainland crewman were arrested and have been charged with attempting to export an endangered species and unmanifested cargo.

Mr Kwok said the 506kg of ivory was the biggest haul since 1992.

He said initial investigations indicated the consignment was destined for Zhuhai, in Guangdong province.

Endangered species protection officer Kwan Sai-ping, of the Agriculture, Fisheries and Conservation Department, said the seizure was the seventh of ivory since 1992 and the largest haul of ivory destined for export.

He added that most seizures were of ivory already cut for jewellery.

'Initial investigations suggest the seized tusks are African elephant ivory,' Mr Kwan said.

He said unworked ivory could fetch about $2,000 a kilogram. However, once it is worked into carvings, chopsticks and jewellery items the price could jump to $20,000 a kilogram.

In Hong Kong, there are about 250 tonnes of marked, licensed ivory. The SAR is a signatory to the 1990 Convention on International Trade in Endangered Species ban on all trade in elephant products.

Last night, the two arrested men were being held at the Customs marine base on Stonecutters Island. They will appear in Western Court today.

The maximum penalty for the illegal trade in a highly endangered species for a commercial purpose is a $5 million fine and two years in prison.