The existence of the Loch Ness Monster was “beyond doubt” in the mind of one of Scotland’s most senior police officers, according to a file released yesterday by the National Archives of Scotland.

William Fraser, chief constable of Invernessshire in the 1930s, was convinced that not only was “some strange creature” bobbing about in Scotland’s second largest loch but it was in danger of being hunted to extinction. He contacted the Under Secretary of State at the Scottish Office in 1933 to raise concerns about the animal’s plight. “That there is some strange creature in Loch Ness now seems beyond doubt,” he wrote. “But that the police have any power to protect it is very doubtful.”

Mr Fraser, who served as chief constable between 1936 and 1951, was responding to a flurry of sightings of a large acquatic beast, or pieces of wood that looked like one. Ministers were sceptical that grainy photographs published by newspapers constituted proof, but questions were asked in the House of Commons as to whether an investigation should be conducted in the interests of science.

Civil servants considered installing “reliable observers” at the side of the loch equipped with cameras to provide definitive evidence of Nessie. Aerial observation was also suggested....