The Queen has become the world's longest reigning living monarch following the death of the King of Thailand.
King Bhumibol Adulyadej of Thailand had reigned since June 9 1946, and was two years younger than the Queen.
Following his death aged 88, Elizabeth II takes up the position as the longest serving current head of state on the planet.
But she is unlikely to acknowledge such a record, particularly as the Thai people are grieving for their revered head of state.
King Bhumibol Adulyadej - whose name means "strength of the land, incomparable power" - had been in poor health for a number of years and was rarely seen in public.
The Royal Palace said the king's health had deteriorated until he passed away peacefully on Thursday at Bangkok's Siriraj Hospital, where a large crowd had been holding a vigil.
Prime Minister Theresa May and Foreign Secretary Boris Johnson joined leaders around the world in paying tribute to the monarch, whose death will be marked with a one-year mourning period by Thai government officials.
Mrs May said: " His Majesty guided the Kingdom of Thailand with dignity, dedication and vision throughout his life. He will be greatly missed.
"Our thoughts are with the people of Thailand at this difficult time."
The king's popularity stemmed partly from his long reign, but he was also viewed as a pillar of stability. Strict lese-majeste laws banned any criticism of him or the Thai royal family.
In August 2015, a man was sentenced to 30 years in prison by a Thai military court for insulting the country's monarchy on Facebook.
The Queen is already the longest reigning monarch in British history and was matter of fact about that achievement.
At around 5.30pm on September 9, 2015, she overtook her great-great-grandmother Queen Victoria, having reigned for 23,226 days, 16 hours and some 30 minutes.
As she thanked the nation for its kind messages as she opened the Borders Railway in Tweedbank on the landmark day, she admitted that the royal record was ''not one to which I have ever aspired''.
She added: ''Inevitably a long life can pass by many milestones. My own is no exception."
The milestone of 63 years and 216 days was tinged with sadness for the Queen, who acceded to the throne at the age of 25, as the calculation of her record is linked to the start of her reign on the death of her father George VI.
She has some way to go before she becomes the world's longest reigning monarch ever.
King Sobhuza II of Swaziland, who died in 1982, holds this title. He was just four months old when he became king and ruled for 82 years and 253 days.
It was estimated that he had about 210 children from at least 70 wives and by the time of his death, he had more than 1,000 grandchildren.
The longest reign of any monarch of a large country in European history belongs to Louis XIV of France - who was also known as Louis the Great, or the Sun King.
His reign lasted for 72 years and 110 days, from 1643 to 1715.
The Queen sent private messages of condolence to Queen Sirikit and Crown Prince Maha Vajiralongkorn, a Buckingham Palace spokeswoman said.