When Harold Carr's nephews and nieces inherited a dusty old lock-up garage from their eccentric uncle their expectations were low.

But when they opened the doors of the car collector's Tyneside garage they discovered what may prove to be a life-changing inheritance. Among a trove of classic cars was a 1937 Bugatti - one of only seventeen in the world - that could fetch millions at auction.

The Bugatti Type 57S Atalante, which had spent decades parked alongside other classics, including a Jaguar E-type and an Aston Martin, will be the highlight of Bonhams' Retro- mobile show in Paris next month.

If bidding reaches upper estimates, it is in the running to become the most expensive car sold at auction, rivalling the 1931 Bugatti Royale Kellner Coupe sold for $8.7 million (£6 million) in 1987. Not bad for a car whose last tax disc expired in December 1960.

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Between then and his death at the age of 89 in 2007, Dr Carr left the sports car to gather dust in his garage, ignoring the dozens of letters from would-be buyers who knew of its whereabouts.

A severe obsessive compulsive who never married or had children, the former surgeon grew increasingly reclusive in his later years. The letters were discovered only when the property was cleared out.

His nephew, an engineer from Gosforth, Newcastle, who asked to remain anonymous, said that while the family knew that the appropriately named Dr Carr had a number of vehicles, none of his relatives had any idea that they were worth such huge sums. “We just can't believe it,” he said. “It's amazing, really. It's worth so much because he hasn't used it for 50 years. People must have known because he got letters from all over the country. He got notes pushed through his door. People travelled to try and convince him to sell the car.”

“He was a very eccentric old gent, I suppose you could call him a mad doctor,” Dr Carr's nephew said.

“All the children would laugh at him in the street when he tinkered with his cars because he wore a piece of rubber tube round his head to stop the oil getting in his hair.

“But he was always such a generous man. It would have been boring and so unlike him to have just left normal things. He was very much a hoarder. In one of his rooms we found many dialysis machines. He used to collect things from hospital skips. He had attempted to repair some of them, and some of the repaired ones he sent to Africa.”

The Bugatti, a black two-seater, was delivered to Earl Howe, the first president of the British Racing Drivers' Club and a winner of the 24 Hour Le Mans race, soon after it was completed on May 5, 1937. He kept the car for eight years, adding personal touches including a luggage rack, after which it changed hands a couple of times before Dr Carr bought it from Lord Ridley, a member of the Northumberland gentry, in 1955.

He drove the car for a few years but by the early 1960s it was parked in his garage, where it remained until after his death. It has exceptional originality, retaining original chassis, engine and drivetrain. Even the odometer reading gives a mileage of only 26,284, despite the vehicle being almost 72 years old.

Dr Carr's nephew said: “It was one of the original supercars. When it was built it could reach 130mph when most cars could only do 50.”

As such, the Bugatti 57S is a magnet for classic car collectors. At least four of the seventeen belong to the French Musée National de l'Automobile in Mulhouse, Alsace, while others remain in private hands.

James Knight, the international head of Bonhams' motoring department, was one of those who knew where the example, chassis number 57502, was hiding.

“I have known of this Bugatti for a number of years and, like a select group of others, hadn't dared divulge its whereabouts to anyone. The Atalante is incredibly original and, although she requires restoration, it is restoration in the true sense of the word. From my perspective, save for some of the interior, all original parts can be restored or conserved in order to maintain originality.

“It offers a truly rewarding project to the new owner to play such an integral part in bringing this wonderful car back to life.

“It has all the finest attributes any connoisseur collector could ever seek in one of the ultimate roadgoing sports cars from the golden era of the 1930s. It is absolutely one of the last great barn discoveries.”

The thrill goes right through your spine

They may cost a fortune to run and steer like an unbroken colt, but nothing matches the magnificance of bowling along country lanes on a summer's day in two tonnes of gleaming black and silver motorcar, the proud owner of 300 veteran, vintage and classic cars said yesterday.

“The thrill goes right through your spine, even at 30mph,” said Lord Montagu of Beaulieu, whose National Motor Museum attracts enthusiasts from all over the world. “It's not like being in a modern car at all. You feel part of the car as you motor along in the fresh air and you can see over people's hedges and gardens, which is great fun. The noise and fumes are all part of it and all the passengers feel part of the travelling experience.”

Lord Montagu, aged 82, who in 1988 drove a 1914 Rolls-Royce across Australia to celebrate the country's bicentenary, still enjoys trundling around his Hampshire estate in one of his 20 roadworthy vehicles. He also looks forward to the annual London-to- Brighton classic car run. “People give you envious looks as you pass by, which is very gratifying,” he said.

Not every classic car owner has a team of mechanics to keep his collection in showroom condition but, according to Lord Montagu, he would be happy to take his part burnishing the coachwork. “All the spit-and-polish work is part of the fun,” he said.

Getting hold of spare parts can be a problem, but even with the oldest models there are ways and means, he said, although he has no plans to bid for the newly discovered Bugatti.

“I am lucky enough to have two already and, unfortunately, we have run out of space in the museum now,” he added.

Styling and sheer power made their marque

— Ettore Bugatti, an Italian, built his first car, the Type 2, in 1901. Exhibiting the four-cylinder model at the Milan Automobile Show, he attracted the attention of Baron de Dietrich, who ordered more than 100 cars, built from 1909 in France. The marque was born

— Dominating motor racing in the latter half of the 1920s, the Type 35 is probably the most famous Bugatti, being the first to feature the iconic arched radiator. Some 96 were produced, winning more than a thousand races between them

— Bugatti also made a motorised railway car and an aircraft, the Bugatti 100P

— The 1931 Bugatti Royale Kellner Coupe is the most expensive car to sell at auction, fetching $8.7 million in 1987. It has a 12.7litre aircraft engine – still one of the largest in a production car

— Volkswagen reintroduced the marque in 2000, offering limited-edition cars in keeping with the tradition of the company. The most recent is the Bugatti Veyron 16.4, which costs more than of £1.2 million