Stretched 8er spotted once again ahead of market launch sometime next year.

The BMW 8 Series Gran Coupe has been spied during development testing, giving us another glimpse at the company's upcoming rival to the likes to the Mercedes-Benz CLS and Porsche Panamera.

Essentially replacing the current 6 Series Gran Coupe, the 8 Series 'GC' should launch sometime next year and will offer a style-focused four-door coupe alternative to the more traditional 7 Series limousine.

In terms of its design, the Gran Coupe looks very much like a stretched version of the two-door 8 Series with the obvious addition of rear doors.

With the extra length, the 8 Series Gran Coupe should offer improved accommodation in terms of both head and legroom for rear passengers, along with increased luggage capacity.

Other than the physical differences, though, we'd expect the four-door 8er to be largely identical to the 'standard' coupe in terms of interior layout, available features and technology, along with powertrains.

That means BMW OS7.0 should be standard for the digital driver's instruments and central infotainment display – measuring 12.3 and 10.25 inches respectively – along with the availability of BMW's latest driver assistance and active safety technologies.

Additionally, the 8 Series Gran Coupe should be offered with a range of turbocharged petrol and diesel engines in six- and eight-cylinder configurations, and the range is also likely to get a range-topping M8 variant, too.

So far, all versions of the two-door 8 Series are equipped with xDrive all-wheel drive and an eight-speed ZF automatic. Expect the same configurations to be available for the Gran Coupe at least from launch, with further variants to follow.

The 8 Series Gran Coupe should be revealed early in 2019 ahead of a market launch later in the year, so expect a debut at next year's Geneva motor show in March. Stay tuned to CarAdvice for all the latest, and head to the galleries section for more images.