The Duke of Edinburgh was admitted to hospital on Wednesday as a "precautionary measure" after the bladder infection he suffered earlier this year returned.
Prince Philip, who had been on his traditional summer break with the Queen at Balmoral in Scotland, suddenly became ill and two private doctors decided he needed further medical treatment. He was admitted to Aberdeen Royal Infirmary where doctors ran a series of tests.
"The Duke of Edinburgh has had a recurrence of the bladder infection that he had in the summer and has been admitted to hospital for investigation and treatment," a statement released by Buckingham Palace said.
"His Royal Highness is likely to remain there for the next few days."
The 91-year-old was forced to miss the Queen’s Diamond Jubilee celebrations in early June due to the same infection and spent five days in hospital.
Despite missing the spectacular Jubilee Concert, he was discharged in time to celebrate his 91st birthday on June 10.
He has since attended the Opening Ceremony of the London Olympics alongside his wife, and watched his granddaughter Zara Phillips compete in the Olympic equestrian teams event.
Despite his family’s increasing worry that he may be doing too much, Prince Philip doesn’t look like slowing down any time soon.
"Prince Philip works an incredibly busy schedule and he is determined to keep doing so. But his family worry he might be overdoing it," The Sun newspaper's royal photographer Arthur Edwards said.
"Everyone has been urging Philip to slow down and take it easy, but he is very head strong. The fact that he continues to work full schedules at 91 is quite amazing. But his determination is being let down by the frailness of his body."
This is Prince Philip's third hospital visit in the past eight months.
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