The 30ft model is thought to have sunk after the shooting of The Private Life Of Sherlock Holmes, directed by Hollywood great Billy Wilder.
It was found on the loch bed during the latest survey of the 755ft (230m) deep stretch of water. A marine robot named Munin is being used to explore areas that have not been reached before.
Experts have said the shape, measurements and location pointed to the object being the prop.
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Project leader Adrian Shine said: “Because Munin can dive and navigate itself safely at great depth, it can approach features of interest and image them at extremely high resolution.
“We already have superb images of the hitherto difficult side wall topography and look forward to discovering artefacts symbolic of the human history of the area.”
Despite no conclusive evidence of the famed monster, the mystery and interest surrounding Nessie is worth an estimated £60 million to the Scottish economy, with hundreds of thousands of visitors travelling to Loch Ness every year in the hope of catching a glimpse.
A spokesman for VisitScotland, which is supporting the project, said: “Operation Groundtruth has uncovered a recognisable creature.
“Although it is the shape of Nessie, it is not the remains of the monster that has mystified the world for 80 years, but a star of the silver screen.”
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Christopher Lee starred in the 1970 film where the monster in fact turned out to be a disguised naval submarine.
The prop is thought to have sunk after its humps were removed and it had not been seen again until now.
Previous discoveries made in Loch Ness include a crashed Second World War bomber, a 100-year-old fishing vessel and parts of John Cobb’s speed record attempt craft Crusader, which crashed at more than 200mph in 1952.
In a further blow to monster hunters, early survey findings have revealed that claims made earlier this year about a “Nessie trench” in the northern basin of the loch are incorrect. More precise underwater evidence shows there is no anomaly or abyss at the location.
The survey - the first of its kind in Scotland - is being carried out over two weeks by Kongsberg Maritime and supported by the Loch Ness Project and VisitScotland.