LONDON (AFP) - Four leaders of a crime gang were convicted on Monday of plotting to steal jade and rhino horn artefacts worth £57 million (S$112 million) from museums across Britain to export illegally to China.

Ten more men have already been convicted of the same crime, including Chi Cheong Donald Wong - a London-based intermediary who would find buyers for the stolen items and made frequent trips to Hong Kong.

A jury in Birmingham in central England convicted the men of planning raids on auction houses and museums, including at the Fitzwilliam Museum in Cambridge.

None of the 18 jade exhibits stolen from the Cambridge museum in April 2012 have been recovered but a jade bowl and figurine taken from Durham's Oriental Museum were found hidden in waste ground.

The men were all members of Rathkeale Rovers, an organised crime gang within the Irish Traveller community.

The European law enforcement agency Europol in 2011 warned about an Irish organised crime group involved in the trafficking of illegal rhino horn.

The police estimate that the loot netted from the thefts would have fetched £57 million in China.

Six members of the gang were arrested in September 2013 at travellers' camps in south east England.

There is high demand for rhino horns in China, where they are used in highly controversial preparations of traditional Chinese medicine.

In recent years, prices of drinking cups made of sculpted rhinoceros horns also have soared in the Chinese art market.