The Duke of Edinburgh has spent a second night in hospital in Aberdeen as he recovers from a recurrence of a bladder infection.
The 91-year-old has been advised to rest and will have no visitors in hospital on Friday, sources have said, but he is understood to be in constant contact with his family by phone while he is being treated.
Prince Philip was taken by ambulance to Aberdeen Royal Infirmary on Wednesday while staying with the Queen at her retreat at Balmoral.
Buckingham Palace said on Thursday that the Duke is responding well to treatment but is likely to remain in hospital for a few more days.
It has since emerged that attempts to airlift the Prince to hospital on Wednesday failed.
A spokesman for the Ministry of Defence said: “We were asked to assist in a patient transfer. We got airborne but due to appalling weather in the area we were unable to assist.”
He said that “extremely poor weather” had hampered the operation with heavy rain and fog in the area.
Pilots from RAF Lossiemouth were able to land the Sea King helicopter three times in fields but were unable to get close enough to the Duke to allow a transfer from the ambulance to take place and the operation was abandoned.
Asked why the helicopter had been scrambled to Balmoral when the Duke had already been collected by ambulance he said: “It depends where they are and what the circumstances are. There are a variety of reasons why we could be asked to assist and if a VIP is poorly that could be one of them. We were asked to assist but were unable to do that.”
Both Buckingham Palace and The Scottish Ambulance Service declined to comment on the use of the helicopter.
At the hospital, a police presence can be seen outside, with four officers standing guard.
The Duke's latest health scare comes 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 celebrations for Her Majesty's 60 years on the throne after he was admitted to the King Edward VII Hospital in central London.
He also underwent a successful procedure to clear a blocked coronary artery in December 2011 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 younger years.
In March, his grandson Prince Harry said the operation had given him "a new spurt of life".
The Duke has recently been seen at a number of public events including the Olympics, where he watched granddaughter Zara Phillips' Olympic equestrian debut, Cowes Week on the Isle of Wight and a tea party at Balmoral where he joined the Queen and the Earl and Countess of Wessex at the event on the estate which was attended by around 3000 guests.