Buckingham Palace last night confirmed the Duke of Edinburgh's weekend engagements had been cancelled after he received treatment for a chest infection at a London hospital.

Concerns for Prince Philip's health were raised after it emerged the 86-year-old royal was taken to King Edward VII's Hospital in central London at 5pm on Thursday by private car after what at first appeared to be a heavy cold.

Although the palace would not confirm last night whether the duke would stay in hospital for a second night, sources said he was unlikely to be discharged so early.

The palace said the prince had been suffering for three or four days with a heavy cold and had shown no signs of improvement after suffering from a chest infection.

But playing down what had happened, the palace said an ambulance was not needed to take Prince Philip for treatment, and it was not an urgent transfer to the hospital. A Buckingham Palace spokesman said doctors want to "work out exactly what is wrong".

The spokesman said Prince Philip had walked into the hospital without assistance and spent Friday working on his correspondence from his hospital bed. The palace later said the Duke of Edinburgh's programme of engagements for the weekend had been cancelled.

Only days earlier, the Queen's husband had looked his normal fit self as he said goodbye to French President Nicolas Sarkozy and his wife Carla Bruni-Sarkozy at the end of their two-day state visit.

He smiled as the Sarkozys' car pulled away and waved to them from the steps of the Sovereign's Entrance at Windsor Castle.

An active man who has enjoyed good health well into his eighties, Philip is known as the Queen's constant companion at public events. Most of his previous ailments and injuries have been sports- related and even as an octogenarian he has continued to compete in carriage-driving competitions. However, he is expected to be monitored closely during his current illness because of his age.

This week a coroner at an inquest into the death of Princess Diana rejected a conspiracy theory that Prince Philip was behind a secret service plot to kill the princess and her boyfriend Dodi Fayed.

The theory has been ridiculed by many, but tenaciously put forward for more than a decade by Dodi Fayed's father, Mohamed al Fayed, the owner of Harrods department store in London.

Lord Justice Scott Baker rejected the idea, saying there was "no evidence that the Duke of Edinburgh ordered Diana's execution and there is no evidence that the Secret Intelligence Service or any other government agency organised it."

Philip had been due to attend a thanksgiving service for Everest conqueror Sir Edmund Hillary on Wednesday but failed to attend because he was taken ill with a cold.

He was also expected to attend a dinner for Macmillan Cancer Support at a hotel in Windsor last night.

A Buckingham Palace spokeswoman said none of his engagements for next week has been cancelled.

On Monday, he is due to attend two engagements at Windsor Castle, a council meeting and a dinner to mark the 60th anniversary of the establishment of the State of Israel.

He is scheduled to accompany the Queen to an engagement at Britannia Royal Naval College in Dartmouth, South Devon, on Thursday.

The Duke of Edinburgh's Award chief executive, Peter Westgarth, said: "The Duke of Edinburgh's Award was concerned to hear about the Duke's illness and wish him a speedy and full recovery."

The duke has also been an advocate of healthy eating combined with exercise.

He does not smoke. He stubbed out his last cigarette shortly before his wedding to the Queen in 1947 having the willpower to give up almost overnight. As he entered his seventies, one of the Queen's former physicians described him as "astonishingly fit for a man of his age".

He has continued to exercise, swimming regularly and taking part in fast-paced carriage driving competitions, which require both stamina and strength. He once said that he more or less followed the Atkins diet and he drank only moderately.

At the age of 82, Philip decided for the first time not to take part in the Trooping the Colour ceremony on horseback. Instead, he travelled in a carriage with the Queen.

He was said to have found previous ceremonies so painful he had to lie on the floor afterwards to recover. The same year, the palace was forced to deny claims that the duke was suffering from prostate cancer.

Those who suggested he was in poor health were given short shrift.

"Do I look bloody ill?" he shouted at one estate worker at Sandringham.