The existence of the Loch Ness Monster was "beyond doubt" according to a top police officer, a historic file released yesterday showed.

The government file about Nessie was released by the National Archives of Scotland as part of the exhibition An Open Secret.

William Fraser, the chief constable of Inverness-shire, raised concerns about protecting the "strange creature" from hunters, in a letter to the Under Secretary of State at the Scottish Office in the 1930s.

He wrote: "That there is some strange creature in Loch Ness now seems beyond doubt. But that the police have any power to protect it is very doubtful."

In 1933, following a number of alleged sightings and the publication of articles and grainy photographs by newspapers, the Scottish Office was asked to confirm the existence of a monster or sea serpent in Loch Ness.

A parliamentary question was tabled in the House of Commons asking whether, in the interests of science, an investigation would be made into the existence of the monster.

Ministers and civil servants were sceptical but, in order to determine whether it existed, it was proposed that "reliable observers" could be stationed around the loch equipped to take photographs.

They also suggested that arrangements could be made for aerial observation.

If the monster's existence was then proven, the next step would be to trap it without injury - but given the size and depth of Loch Ness this could prove a difficult task.

In the end it was felt that as the monster provided public interest and amusement, it would be better to let it continue to do so than to kill it, according to the archive.

But this did not stop monster hunters from frequenting the shores of Loch Ness in the hope of seeing the monster, capturing it or harming it.