The Prince of Wales asked the US if it would be possible to pause the invasion of Afghanistan to honour Ramadan, according to Washington's former ambassador to Britain.
William Farish said Charles asked if his request for the holy month to be considered as part of the battle plans could be passed on to the then president George Bush.
The former diplomat recalled the alleged conversation, said to have taken place by phone around a month into the operation, for a new biography of Charles serialised by the Daily Mail.
"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," Mr Farish said.
When it was explained that it would be difficult to halt the military operation already under way, Charles allegedly replied: "But Americans can do anything!"
Mr Farish, 78, served as 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 9/11 attacks.
Charles also attended the service around seven weeks after the Afghan invasion began on October 7 2001.
Ramadan began on November 16 and ended on December 17.
Prince Charles: The Passions And Paradoxes Of An Improbable Life, by Sally Bedell Smith, is published by Random House and will be released on April 4.