The grainy black and white images of a dark blob moving slowly across grey water was hailed in cinemas across Britain as final 'proof' of the Loch Ness monster's existence and sparked nationwide Nessie mania that has continued until this day.

But in the 65 years since it was first shown, the short newsreel clip of the 30ft long creature, "almost black in colour and very shiny" has proved as elusive as the beast.

Now the unique footage - first shown in 1936 - has been found and is set to be screened for the first time on television this St Andrew's night.

Janet McBain, curator of the Scottish Screen Archive, found the 16mm film dumped in an old rusty tin amongst hundreds of film cans passed on to the organisation by the former Scottish Film Council.

The short movie was made on September 22, 1936 at Loch Ness by Glasgow filmmaker Malcolm Irvine for his Scottish Film Productions Company. Shown in cinemas around the country in what was a forerunner of Pathe News, with the title: 'The Loch Ness Monster - Proof At Last', it sparked the Nessie legend.

McBain said: "The existence of the film was well documented at the time but it disappeared without trace and most people thought that it had been destroyed and lost for ever.

"But when the company went out of business in the 1930s they donated all their old footage to the Scottish Film Council. The council, in turn, appears to have dumped it in an old store along with about 20,000 other old cans of films and forgotten about them.

"The cans were eventually passed on to Scottish Screen Archive and we have gradually been working through the backlog, viewing, cataloguing and discovering exactly what is in each tin."

She added: "The find is even more remarkable because about 15 years ago I talked to a very old woman who worked for Irvine and was with him the day the film was taken. She told me a remarkable story. Irvine had in fact first seen the monster for the first time three years previously but his camera jammed and he only had a few seconds of footage."

Irvine spent three weeks at the loch side, working with cameramen Stanley Clinton and Scott Hay, before he got the footage he wanted. Armed with a 16mm camera fitted with a long lens, they filmed the monster on the east side of the loch about 100 yards from Inverfarigaig, opposite Urquhart Castle.

On the old original film, Irvine says: "We were so excited and elated when the monster appeared. What you see on the screen lasts less than a minute, but it seemed hours when we were making it. It definitely is something with two humps - that much is clear".

Iain McMillan, another eyewitness who appears in the film says: "We first saw its head and neck, then two humps one behind the other, and then something thrashing around behind from side to side, like a tail."

When the film of the creature was shown to the Linnaean Society, a body which classified animals, no one could give an explanation as to what it could be. Since 1936 there have been 27 more recorded films taken of Nessie and hundreds of other officially recorded sightings.

Today the Highlands of Scotland tourist board says the pulling power of Nessie is "incalculable" although the monster is a worldwide tourist attraction, bringing over £120m a year into the local economy.

The most common area for sightings of the monster is close to the ruins of Urquhart Castle at Drumnadrochit, which now attracts over 200,000 visitors a year.

Fraser Cameron, who runs the Drumnadrochit Hotel and the castle tearoom said: "I know a lot of very reliable people who have seen the monster, including a police chief who watched it for nearly an hour.

"The monster is the only industry we have in the area and it is growing. This summer there has been no drop off in the number of Americans and Europeans from all walks of life who have come hoping to see Nessie. We are even welcoming Russians in growing numbers and Croatian bus tours".

Malcolm Irvine's film of the Loch Ness Monster will be screened in a special St Andrew's night programme, Scotland on Film, at 10pm on BBC Two.