In a message to the late king's brother Crown Prince Tupouto'a Lavaka – who will succeed him as king – the monarch said: "It is with great sadness that I learnt of the death of your brother, His Majesty King George Tupou V.
"King George was a true statesman who served his country with distinction.
"On behalf of the people of the United Kingdom, Prince Philip and I wish to convey to your family and to the people of the Kingdom of Tonga our deepest sympathy at this sad time."
A state funeral will be held for the late monarch on the island of Nuku'alofa – Tonga's capital – on March 28.
It is expected to be a lavish affair, attended by several foreign dignitaries.
King George died in hospital on Sunday in Hong Kong.
No cause has been given for his death, though it is thought to be related to complications with his kidneys – he had one removed earlier this year after a tumour was discovered.
A government spokesman has said Tonga – part of the Commonwealth, and made up of 176 islands in the South Pacific - has declared an official period of mourning until June 19.
During this time, Tongans are encouraged to wear black and refrain from loud entertainment and celebrations.
Oxford-educated King George, who never married but had a daughter out of wedlock, was known as a colourful character.
He had a penchant for dressing in military regalia, and was often seen driving around in a London black cab.
The monocle-wearing monarch also enjoyed sailing miniature boats in the palace swimming pool, and staging murder mystery parties for his friends.
Despite his eccentricities, King George promised his people he would help bringing about democracy.
Two years after his 2008 coronation feudal rule officially came to an end when Tongans were able to vote in their first elected parliament.