Over the past year, BMW has been testing a rather strange prototype based on an 8 Series, complete with louvres cut into the rear side windows. At the time, rumours were rife of it being a mid-engined mule for a future supercar, although Munich was quick to deny it.

Now, our spy photographers have caught a second prototype without virtually zero camouflage, revealing the front and rear fascias and a number of mean performance goodies. These include gaping double kidney grilles with menacing gradated red inserts – a colour that also appears on the front air intakes and front fender Air Breather vents. There’s also a jutting front splitter, a massive rear wing and a still-disguised rear diffuser with a Formula 1-style centre fog light.

All this has led our snappers to believe that this is in fact a mule for a possible CSL version of the M8, but there are a few problems with that theory. First of all, the car is quite obviously based on the standard 8 Series, as evidenced by the regular M Sport front bumper, side skirts and narrower front fenders compared to the full-fat M car. Only the rear bumper is recognisably from the M8.

Elsewhere, the car rides on smaller 19-inch alloy wheels (the rear wheels come from the Z4 M40i), whereas the M8’s wheels measure 20 inches in diameter. But the most glaring cue that this isn’t an M8 are the brakes, which are the standard M Sport units (albeit with cross-drilled discs from the M Performance Parts catalogue) rather than the M car’s massive stoppers.

This leaves us with two possibilities of what this car could be. The first is the original theory that this is an early mule for a mid-engined M-badged supercar, using 8 Series running gear. This remains an outside possibility, given that BMW showcased the Vision M Next concept in 2019, utilising a four-cylinder turbo engine and an electric motor on each axle to deliver a total system output of 600 PS. However, recent reports suggest that the project has been cancelled due to the coronavirus pandemic and spiralling costs.

The other option is that this is simply a development vehicle to test various components, such as new chassis and engine parts, tyres, brakes, exhausts and infotainment systems. That was what a BMW official told Auto Motor und Sport when asked about the mule, and could very much still be the case.

Even if this isn’t a prototype of the M8 CSL, a hardcore version of the performance coupé is said to be still on the cards, due to arrive next year as part of BMW M’s 50th anniversary celebrations. Interestingly, the rumour mill points to the company following in the footsteps of Mercedes-AMG in switching to hybrid power, ditching the M8’s 4.4 litre twin-turbocharged V8 for a 3.0 litre straight-six. Total system output is expected to push past 700 PS, which would make for quite a potent machine if the reports are to believed.