BMW will broaden its 8 Series to three bodystyles with a four-door coupe this year – and photos of the new machine have leaked online ahead of its reveal.

The Porsche Panamera rival is due to launch shortly, with BMW recently issuing the first official teaser image, but new leaked pictures show the car in full. First posted to the Bimmerpost forum, they show how the 8 Series has grown in size to accommodate the extra doors, with a newly elongated rear. The shape is still recognisable from the two-door car, though.

The new car takes over the market position previously occupied by the old 6 Series Gran Coupe and is directly aimed at cars such as the Porsche Panamera and Mercedes-Benz CLS. It shares its engines and underpinnings with the 8 Series Coupe, but with an extended wheelbase and raised roofline offering more passenger and luggage room.

Previous spy images also showed the design is a familiar adaptation of the coupe and convertible, with the undisguised front end looking largely identical and a recognisable rear shape. It's not clear yet, however, whether two or three rear seats will be offered; the coupe is a strict two-seater in the back.