The Dawson notes, now preserved in the archives of Windsor Castle, were first examined by the physician's biographer, Francis Watson, when he prepared a volume that appeared in 1950, five years after Lord Dawson's death. At the request of the physician's widow, the biographer said today, he simply omitted any reference to the euthanasia that had taken place at Sandringham Castle.

It was Mr. Watson, now 79 years old, who filled in the omission in an article in a journal called History Today that went on sale here this morning. ''Perhaps I should have included it in the book at the time,'' he said. ''Lady Dawson did not want it in the book and I quite readily agreed. I didn't think it appropriate.'' Queen's Reaction Not Known

The reaction of Queen Elizabeth II, the granddaughter of Lord Dawson's patient, could not be learned. ''It happened a long time ago,'' the spokesman at Buckingham Palace told callers, ''and all those concerned are now dead.''

Lord Dawson's notes assert that he had been told by Queen Mary and the Prince of Wales - the playboy son who was to become Edward VIII and, less than a year later, would abdicate and become the Duke of Windsor - that they did not want the King's life needlessly prolonged if his illness was clearly fatal. There is no indication that the King himself had been consulted.

It is not clear from the notes how explicit Lord Dawson was in the exchange he reported with the Queen and Prince about the method of ending the King's life, or whether this conversation had been initiated by the family or the physician. But there is circumstantial evidence, in a speech Lord Dawson delivered in the House of Lords in a debate on euthanasia 10 months later, to suggest that the discussion could have been prompted by the doctor. 'Mission of Mercy'