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The touching letter was kept by Logue in a silver cigarette case also gifted from the King.
A heartfelt private letter of gratitude from George VI to his speech coach who was immortalised in the film 'The King's Speech' has come to light after 83 years.
The monarch, who had a stammer from his childhood, wrote to Lionel Logue to thank him for helping him prepare for his 1937 coronation speech.
In the previously unseen letter the King tells his Australian therapist of his 'anxiety' in the lead up to the historic occasion, which was exacerbated by a 'poor rehearsal', and his overwhelming relief at the speech having gone smoothly.
He then confides in Logue, who he had worked with for 11 years, that he would not have managed it without his 'expert supervision and unfailing patience', adding he 'truly didn't know how I could have done it without you'.
Their friendship was the subject of the 2010 film, in which Colin Firth won an Oscar for his portrayal of George VI starring opposite Geoffrey Rush who played Logue.
The letter, and the silver cigarette case it was enclosed in, are being sold with auctioneer Woolley & Wallis, of Salisbury, Wilts, who expect the items to fetch £4,000.