FEARS have grown for the Duke of Edinburgh's health after he was admitted to hospital for the third time in less than a year.

It was announced last night that he would undergo stomach surgery today.

A Buckingham Palace spokewoman confirmed Prince Philip had been admitted to the London Clinic for an exploratory operation under general anaesthetic following abdominal investigations carried out over the last few days. She added that he was in good health.

The Duke, who will be 92 on Monday, is expected to stay in hospital for up to two weeks.

He pulled out of engagements in May and this month on health grounds. However, the Prince's illness earlier this week is not thought to be connected to his latest admission.

The spokeswoman added: "He felt unwell on Monday and missed an engagement but that was down to the fact he had temporarily lost his voice."

The news came just hours after the Queen and Prince Philip had appeared together on the steps of Buckingham Palace. They were accompanied by the National Anthem to mark the start of a garden party at 4pm.

Despite the fierce sun, with temperatures at about 22˚C (72˚F), the Duke appeared relaxed and happily chatted to guests as he walked around the garden.

Lord-Lieutenant of Nottinghamshire Sir John Peace, who attended yesterday's garden party, said Philip had been telling jokes.

Speaking of meeting the Queen at the event, he added: "It was wonderful to see the Duke of Edinburgh with her."

The Duke spent four days in hospital during Christmas 2011 after he collapsed suffering chest pains on Christmas Eve at Sandringham. He subsequently underwent surgery to clear a blocked artery, which was said to have given him a "new spurt of life".

However, in June last year he was back in hospital after becoming ill with a bladder infection shortly after braving heavy rain during the Jubilee boat pageant to mark the Queen's 60-year reign, and was forced to miss several days of festivities.

The problem recurred again in August during the royal family's annual holiday at the Balmoral Estate in Aberdeenshire, and the Duke was taken by ambulance to Aberdeen Royal Infirmary for treatment. He was released five days later.

The Duke had missed an engagement at the palace on Monday night as he was feeling "under the weather", although he joined the rest of the royal family at Westminster Abbey on Tuesday for the official service marking the Queen's 60 years as monarch.

He is the longest-serving royal consort in UK royalty.

The Palace said in a statement: "His Royal Highness The Duke of Edinburgh has been admitted to the London Clinic for an exploratory operation following abdominal investigations.

"He is expected to stay in hospital for up to two weeks.

"Further updates will be issued when appropriate."

Prime Minister David Cameron sent Philip his good wishes.

He wrote on Twitter: "My best wishes to the Duke of Edinburgh who is in hospital tonight. I hope he has a swift recovery."