"It was (in) Hong Kong, and the Crown Prince Tupouto'a Lavaka was there at the hospital just before the king passed away," the brief Matangi Tonga Online report said.

"This news is yet to be officially confirmed but our reliable source informed us that the king was rushed to hospital early this afternoon but it is understood that he had passed away a few hours later."

Government and royal palace officials in Tonga were not immediately available to confirm whether the 63-year-old monarch had died.

Tupou V, a strong advocate for democratic reform in the tiny South Pacific kingdom, was sworn in as king in September 2006 following the death of his father king Taufa'ahau Tupou IV.

However, his coronation was delayed until 2008 as the impoverished Polynesian nation of 115,000 reeled from the impact of riots in the capital Nuku'alofa.

Eight people were killed and much of the central business district was destroyed in the November 2006 riots when people protested against the slow pace of political reform.

Within two years after his coronation, Tupou V made good on his pledge for democracy and the people of Tonga were able to vote in their first popularly elected parliament.

The democratic changes replaced the former feudal system where the kingdom was run by a parliament dominated by a clique of nobles selected by the king, who also chose the prime minister and cabinet.

Outside Tonga, the king was known for the elaborate uniforms, colonial-era pith helmets, and monocles that he wore and for being driven around the capital in a London cab.

The king, who was not married, went to primary school in Switzerland and went on to study at Oxford University and the Royal Military Academy Sandhurst in England.