Loch Ness Monster exhibition in 1980, interviewed Mrs Mackay, then in her eighties, alongside BBC journalist Nicholas Witchell in 1986, shortly before her death.

He recalled: ‘I clearly remember we were standing by the side of the loch when Nick asked her how big the thing was. She thought about it carefully and said, “It was enormous. At least 6ft, maybe nine.”

‘I couldn't help myself and said, “Oh, Mrs Mackay, why did you say it was enormous, then?” She was quite indignant and said, “Well, I fish for salmon”. Mr Harmsworth added: ‘She couldn't believe how the whole thing had taken off. She got fed up of talking to people about it.

‘It's incredible that one woman sparked off the hunt for the Loch Ness Monster.’

In his newly published book Loch Ness Understood, he claims the first ever photograph of Nessie was literally a shaggy dog story.

Aluminium worker Hugh Gray's photograph made the front page of newspapers around the world after it was taken when he spotted a commotion in the waters of the loch in Novemeber 1933.

Mr Harmsworth said: ‘We had the photo on display at the exhibition and one day a visitor told me with absolute confidence that it showed a labrador dog.

‘When I looked again, I couldn't believe my eyes. There was a Labrador swimming towards the camera with a stick in its mouth. It became obvious and from then on I became convinced that's what it is.’

There have been more than 1,000 recorded sightings of the monster, which has become the Highlands' most lucrative attraction, inspiring hit Hollywood films Loch Ness and The Water Horse.

Mr Harmsworth will appear in Nessie at 80, part of the Edinburgh International Science Festival, on April 6.