The Bentley 4 1/2 litre was built in 1929 and, six years later, bought by the grandfather of the current owner.
He would take the luxury saloon on family holidays around the UK, even towing a caravan behind.
The car was taken off the road in 1985 by the family and parked in a barn in Surrey where it remained until six weeks ago.
The current owner, who does not want to be named, thought it would be worth around 30,000 pounds.
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"To collectors this really is the holy grail. The chassis, engine, axle, gearbox, steering box numbers are all present and correct," Hubbard said.
"I have been doing this job for ten years and to find a car like this is fantastic. It will need to be looked at but the body and trim are in remarkable order and won't need to be replaced," he said.
Bonhams has estimated that the car may fetch between 150,000 pounds and 250,000 pounds, 'express.Co.Uk' reported.
"The owner thought it might be worth 30,000 pounds so it was great to tell him it could be worth ten times as much," he said.