The prototype car hit 186 mph in 1981.

Stories behind barn find cars are always interesting, but there's an extra level of intrigue with this one.

The BMW M1 is already a rare car, but this particular M1 is something rather special. Built and delivered as a 'standard' mid-engined supercar in 1979, two years later racing driver Harald Ertl got his hands on the car and get about giving it an extensive rework, which included the fitting of twin KKK turbochargers tot he 3.5-liter six-cylinder engine, upping the horsepower figure from 273 to 410 horsepower.

The car was developed in conjunction with BP in a bid to promote its Autogas product. The aim was to break the 300 kph (186 mph) barrier. That figure is said to have been achieved on October 17, 1981, however, there are no official records to support the claim.

The wacky-looking record-breaker has been 'lost' for nearly 25 years, but now it's hitting the market for the first time since 1993. It is expected to fetch as much as £250,000 (327,000) at the Coys auction at Techno Classica in Essen, Germany on April 13th.

"This is undoubtedly a piece of motoring history and possibly one of the rarest BMW’s to carry the M1 badge," said Chris Routledge, managing director at Coys. "There is huge interest from around the world from people who wish to own a piece of motoring and speed record-breaking history."

"This is as a result of our untiring detective work in the world of classic cars which has resulted in the barn finds we have unearthed, from the lost Ferrari 250 GT to the Cavalino treasure trove of parts which included valuable spares and items from the great Italian marques such as Ferrari, Maserati and Abarth," he added. "The BMW is another astonishing barn find."

Source: Coys

Gallery: 1979 BMW M1 World Land Speed Record