A marine robot deployed in the waters of Scotland's Loch Ness has found the remains of a monster but it turned out to be a prop from a movie shot in 1970.

The robot, belonging to Norwegian offshore oil company Kongsberg Maritime, is drawing up the first high-resolution map of the 230-metre (755-feet) deep lake in a project named "Operation Groundtruth".

"Although it is the shape of Nessie, it is not the remains of the monster that has mystified the world for 80 years," Scottish tourism agency VisitScotland, which is backing the project, said on Wednesday.

The agency's statement said "Nessie found" with an asterisk at the bottom reading "replica model".

The blurry object with a long neck was a 30-foot (9.15-metre) long model of the monster made for the film "The Private Life of Sherlock Holmes", directed by Billy Wilder.

"It is thought the model sank after its humps were removed (the buoyancy was in the humps) never to be seen again," VisitScotland said in a statement.

The monster was actually a submarine in the film.

The mapping is being carried out by a robot called "Munin", which resembles a missile-shaped drone.

It also found a 27-foot long shipwreck, which is still being investigated, and worked out that there is no "Nessie trench" in the loch bed in which a creature could be hiding, as previously believed.

"The vehicle is providing insight to the loch's depths as never before imagined. Finding Nessie was, of course, an unexpected bonus," Craig Wallace, a Kongsberg Maritime engineer, said in a statement.

Previous discoveries made in Loch Ness include a crashed World War II bomber plane, a 100-year-old fishing boat and the remains of a speedboat used in a 1952 speed record attempt which killed its pilot.

The lake has been notoriously difficult to survey due to its depth and steeply sloping side walls.

VisitScotland estimates the revenue generated by tourists hoping to catch a glimpse of "Nessie" at £60 million (76 million euros, $85 million) a year.