The Duke of Edinburgh, 99, is expected to spend a third night at King Edward VII Hospital in London, after being admitted on Tuesday.
In a statement released on Wednesday, Buckingham Palace said: "The Duke’s admission is a precautionary measure, on the advice of His Royal Highness’s Doctor, after feeling unwell.
"The Duke is expected to remain in hospital for a few days of observation and rest."
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WATCH: Prince Philip, 99, admitted to hospital as 'precautionary measure'
HELLO! understands that Prince Philip, who is said to be in "good spirits", walked into hospital by himself after travelling by car and that it was not an emergency admission. The stay in hospital is purely precautionary and the illness is not COVID-19 related.
The Queen has remained at Windsor Castle where she and her husband have been spending lockdown. On Wednesday, she carried out an engagement, speaking by phone to Admiral Tony Radakin, the First Sea Lord of the Royal Navy. The royal family's Twitter account confirmed her virtual call shortly after Philip's hospitalisation was announced.
The monarch will celebrate her 95th birthday in April, while Philip is set to mark his 100th birthday on 10 June.
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Philip's last public appearance in July 2020
Prince Philip's recent public appearances
The Duke, who retired from royal duties in August 2017, made his first public appearance in over a year last July as he handed over the role of Colonel-in-Chief of The Rifles to the Duchess of Cornwall virtually.
Prince Philip took part in a ceremony at Windsor Castle, while Camilla, 73, was at Highgrove House in Gloucestershire.
The Queen and Prince Philip pictured on the Duke's 99th birthday
He and the Queen also attended Princess Beatrice's wedding to Edoardo Mapelli Mozzi in Windsor in July 2020 and were photographed beaming proudly at the newlyweds in the official photographs.
The monarch and her husband celebrated their 73rd wedding anniversary in November, with Buckingham Palace sharing a new photo of the couple smiling at a handmade card from their great-grandchildren, Prince George, Princess Charlotte and Prince Louis.
To mark his 99th birthday in June 2020, Buckingham Palace also shared a new portrait of the Duke with the Queen in the grounds of Windsor Castle.
Prince Philip with Prince Harry at Lady Gabriella Windsor's wedding in 2019
Philip also attended the weddings of his grandchildren in 2018, when Prince Harry married Meghan Markle, and Princess Eugenie tied the knot with Jack Brooksbank in Windsor in May and October respectively.
The Duke also made a last-minute appearance at Lady Gabriella Windsor and Thomas Kingston's nuptials in May 2019.
Prince Philip's health
Philip has suffered a number of ailments over the years, including being treated for a blocked coronary artery at Papworth Hospital in Cambridgeshire in December 2011 and a bladder infection in June 2012, forcing him to miss the Queen's Diamond Jubilee celebrations.
Exploratory surgery on his abdomen followed in June 2013 and he began to use hearing aids by 2014, aged 93.
Prince Philip's last official public engagement before his retirement in 2017
He later pulled out of a Battle of Jutland anniversary event in June 2016 citing a minor ailment which was followed soon after by his retirement from public duty in 2017.
He spent four nights at King Edward Hospital in December 2019, where he was treated for a "pre-existing condition" and was later discharged on Christmas Eve.
In January 2019, the Duke made headlines after he was involved in a car crash on the Sandringham Estate. His Land Rover Freelander collided with another vehicle, leaving the two women in the Kia with minor injuries. Philip's car overturned but he managed to escape unscathed.
In February 2019, it was confirmed that Prince Philip, then 97, had voluntarily surrendered his driving licence.
The royal family's go-to hospital
Many a royal has been cared for at the private King Edward VII's Hospital. From the Queen to the late Princess Margaret and the Queen Mother, the exclusive clinic in central London has been the first port of call for members of the Firm for years.
The Duke of Edinburgh has been admitted a number of times in recent years and the Duchess of Cornwall had a hysterectomy at the medical institution in 2012.
The first time the Queen was admitted to hospital was at the King Edward VII's in July 1982 when she had a wisdom tooth extracted.
Kate leaving the hospital after treatment for severe morning sickness in 2012
The Duchess of Cambridge was also treated at the hospital for hyperemesis gravidarum (severe morning sickness) when pregnant with Prince George.
King Edward VII's Hospital was established in 1899 by two sisters, Agnes and Fanny Keyser, who turned their home at 17 Grosvenor Crescent into a hospital for sick and wounded officers returning from the Boer War.
The hospital moved to its present site in Beaumont Street in 1948, and in 2000 it changed its title to King Edward VII's Hospital Sister Agnes.
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