Prince Charles tried to halt the invasion of Afghanistan to respect Ramadan, Washington's former ambassador to Britain has claimed.
The Prince of Wales asked that his request for a pause in the military action to honour the Muslim holy month could be passed on to then president George W Bush, according to William Farish.
The former diplomat said the conversation took place by phone around a month into the operation.
In a new biography of the heir to the throne, serialised by the Daily Mail, Mr Farish said: "Prince Charles asked me if it would be possible to stop the invasion to honour Ramadan, and if I could convey that request to President Bush."
It is claimed when Prince Charles was told that it would be difficult he replied: "But Americans can do anything!"
According to the account, when the envoy asked: "Sir, are you really serious?", Charles replied: "Yes I am."
Around 20,000 American and British troops were sent into Afghanistan by President Bush and then Prime Minister Tony Blair in October 2001 after the ruling Taliban refused to hand over al Qaeda leader and mastermind of the 9/11 terror attacks Osama Bin Laden
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Charles is said to have made his request about a month into the operation and two weeks before the start of the holy month.
Mr Farish,78, was US ambassador to the UK between 2001 and 2004 and appeared alongside George Bush senior, Tony Blair and the Queen at a service to remember the victims of the attacks on the Twin Towers in New York.
Prince Charles also attended the service around seven weeks after the Afghan invasion began on 7 October, 2001.
Ramadan began on 16 November 16 and ended on 17 December.
The claims emerged as Prince Charles was due to honour First World War dead during a visit to Italy, while the Duchess of Cornwall visits a project supporting vulnerable people, based in a former mafia villa.
The Royal couple are touring Europe on what has been widely interpreted as a Brexit diplomacy visit, aimed at strengthening ties as the UK begins the process of leaving the EU.
Prince Charles: The Passions And Paradoxes Of An Improbable Life, by Sally Bedell Smith, is published by Random House and will be released on 4 April.