Duke of Edinburgh in hospital for fourth day as he continues treatment for bladder infection

The Duke of Edinburgh remained in hospital for a fourth day today as he continues his treatment for a bladder infection.

Philip, 91, was rushed to Aberdeen Royal Infirmary on Wednesday while staying with the Queen during their annual summer break at Balmoral.

An RAF search and rescue helicopter was immediately dispatched to fly him to the hospital but poor visibility and heavy rain meant the crew, who are used to working in difficult conditions, were unable to reach him.

The Duke made the journey of almost 50 miles from the castle in the Cairngorms to the city hospital by road in an ambulance instead.

Doctors advised the Duke to rest yesterday, meaning there were no visits from other members of the Royal Family.

But Philip is said to have been in constant contact with his relatives by phone.

He is expected to remain in hospital for the next few days and is said to be responding well to treatment for the recurrence of the infection he suffered in the summer.

The Sea King landed four times during the expedition on Wednesday, but could not get close enough.

"The weather was absolutely dreadful," a source said.

A Ministry of Defence spokesman said: "The RAF can confirm that a search and rescue crew from RAF Lossiemouth in Morayshire was asked to assist with the transfer of a patient.

"However, due to poor weather we were unable to assist."

At the hospital, a police presence could be seen outside, with four officers standing guard.

The Duke's latest health scare came two and a half months after he spent five nights in hospital with a bladder infection following the Diamond Jubilee Pageant on the River Thames.

He was forced to miss some of the key celebrations for the Queen's 60 years on the throne after he was admitted to the King Edward VII Hospital in central London the day after braving wet and windy elements on a barge for several hours.

Philip also underwent a successful procedure to clear a blocked coronary artery last December and was in hospital for four nights over Christmas.

Before the heart scare he had been fit and apparently healthy, and led the active life of a man of fewer years.

In March this year his grandson Prince Harry said the operation had given him "a new spurt of life".

Philip went to Balmoral, where the royals often retreat during the summer, after attending a number of Olympic events. He was at the Queen's side during the Opening Ceremony on July 27 and the following day when they toured the Olympic Park.

He then watched his granddaughter Zara Phillips' Olympic equestrian debut on July 29 and was at the ExCeL exhibition centre a few days later to watch boxing.

The Duke appeared to be in good spirits as he took part in events as part of Cowes Week on the Isle of Wight, in his role as Admiral and member of the Royal Yacht Squadron.

He viewed the racing in the Solent on Tuesday before attending a luncheon on board a yacht, the My Farragon, accompanied by the Lord Lieutenant of the Isle of Wight, Major General Martin White.

On Monday he had been at a luncheon held at Newport's Chamber of Commerce, then visited manufacturers AJ Wells and Sons in the town before having dinner at the Pavilion in Cowes.

While in Scotland earlier this month he supported the Queen at engagements including hosting a tea party at Balmoral.

Philip joined the Queen and the Earl and Countess of Wessex at the event on the estate, which was attended by around 3,000 guests.

The royals, including the Duke, reportedly looked relaxed during the event, which rounded off the Queen's Diamond Jubilee celebrations.

The next official engagement for the Duke listed in the royal diary is accompanying the Queen to the opening of the London 2012 Paralympic Games on August 29.

It is not yet known whether he will attend.