This unique Alfa Romeo is very special indeed, being the only prototype built for a planned limited series of the Alfa Romeo 155 GTA Stradale to celebrate Alfa Romeo's success in touring car racing where the 155 was immensely successful in touring car racing across Europe, winning the Italian Superturismo Championship and German DTM Championship (both with Nicola Larini at the wheel); the Spanish Touring Car Championship (with Adrián Campos); and the British Touring Car Championship (with Gabriele Tarquini). Its “Tipo” 156 replacement would prove equally competitive.

Following the success of the “Tipo” 155 competition cars, Alfa Romeo decided to produce a “Stradale” version, similar to the successful Mercedes-Benz 190 E 2.5-16 and BMW M3 models. The 155 GTA Stradale project commenced under the supervision of legendary Abarth engineer and 'father' of the Lancia 037, Sergio Limone.

Mechanically, the prototype is based on the 155 Q4, the sportiest of the 155 range, which uses basically the same engine and mechanicals as the Lancia Delta Integrale, though with the rear differential casing in cast iron (not aluminium) to shift the weight bias rearwards.

The engine was prepared to Group N specification and the body modified with larger wings, race-style front and rear bumpers, and a large rear spoiler. Also, the suspension was changed and is similar to that of the Lancia Delta Integrale, while the 'sports/luxury' interior was trimmed in black leather with 'anatomic' sports seats.

But the Fiat bosses saw two major problems: one was that they would have preferred a more powerful six-cylinder engine, which would not have fitted the compact Delta Integrale drivetrain, and the second was the relatively prohibitive cost, as a separate production line would be needed.

Hence the plan was quickly abandoned and the completed car – which is set to be be offered by auctioneer Bonhams as “Lot 23” during its high profile Padua sale on 27 October – remains the only prototype for the project that was ever built.

The prototype – chassis number ZAR16700000005892 – was displayed at the 1994 Bologna Motor Show and was also used at the Italian Grand Prix that same year as a medical car, driving the legendary British neurosurgeon and FIA medical officer, Dr. Sid Watkins, better known in motorsport by his nickname 'Professor Sid'.

It subsequently ended up in Tony Fassina's workshop in Milan, where it remained for four years before being purchased by one of Mr Fassina's friends. The latter then brought the car to Germany where it was road-registered for the first time.

In 1999, the Alfa returned to Italy, forming part of the private collection of an Alfa Romeo engine preparation specialist and enthusiast in the Marche region until it changed hands, passing to its current owner only recently.

Its condition testifies to this car's careful ownership thoughout its life. It remains outstandingly original with the exception of the rear spoiler, which was reconstructed recently.

This Alfa recently featured on the front cover of the renowned Italian classic car magazine, Ruote Classiche (May 2018 edition), in whose article Sergio Limone vividly remembers the car, saying it was a 'missed opportunity' and should have gone into production.

Now displaying a total of some 40,000 kilometres on the odometer, it is described by the vendor in good order throughout.

Accompanying documentation consists of Italian registration documents: various documents pertaining to its history; a copy of the aforementioned Ruote Classiche; and a letter from Sergio Limone to the then owner Tony Fassina testifying to its originality. Bonhams estimate the 155 GTA Stradale will fetch between 180,000 - 220,000 euros.

Photos: Alfa Romeo 155 GTA Stradale Prototype