BMW is poised to reveal its new 3-series GT later this week, ahead of its Geneva motor show debut.

Undisguised pre-production prototypes of the GT, the third model in the new 3-series lineup after the saloon and estate, have already been spotted on the roads around BMW’s Munich headquarters. They have revealed a car with more resolved looks and proportions than the larger 5-series GT.

Insiders have also divulged much of the BMW 3-series GT’s technical secrets ahead of its official unveiling online tomorrow (Thursday). At around 4825mm in length, the new model is set to be some 200mm longer than the 3-series Touring on which it is based, with 110mm of that increase within the wheelbase. Height is set to increase by around 80mm to 1510mm, with width getting a small 20mm gain to around 1830mm.

Those increased dimensions result in a more spacious cabin. The driving position is raised by around 60mm for a more commanding view of the road, while legroom for rear passengers has been increased by as much as 70mm.

Boot capacity is said to trump the 3-series Touring by around 25 litres at 520 litres.

The engine range, which will consist of five choices at launch, is understood to focus on the more powerful units from the 3-series range. Petrols will include a turbocharged 2.0-litre four-cylinder in two states of tune in the 320i and 328i, and a 3.0-litre, six-cylinder turbo in the 335i range-topper. A 2.0-litre turbodiesel will be offered in two different states of tune via the 318d and 320d.

The 3-series GT is set to reach the UK in late spring, commanding premiums of around £1500 over the equivalent 3-series Touring.