It’s no secret that Mercedes’ luxury R-class MPV has “had a slow start”, as Benz sources admit. That’s very bad news for German competitors planning rival models.

Remember the Audi Roadjet concept from January’s Detroit show? Ingolstadt is forgetting about it, having put plans for a production version of the jacked-up, high-roof A4 on ice. But its sister company, Volkswagen, is pushing ahead with plans for a sleek and sexy MPV, to take on the R-class and Ford’s S-Max from 2009.

Audi is integral to the project. The five-door hatchback is set to be spun off today’s A6 Avant, which is pensioned off in 2010. That means a big car close to 4.9m in length, although the current thinking is for only five seats in two rows, and heaps of luggage capacity. Think of it as a luxurious, spacious MPV that fits between the Touran and Sharan, with prices around £20,000.

Power comes from four- and six-cylinder petrol and diesels. VW will get access to the 2.7- and 3.0-litre V6 TDIs that are currently the preserve of Audi, as well as the 280bhp 3.6-litre V6 petrol heading for the Passat R36.

BMW also plans an R-class rival in 2009, dubbed ‘super-Touring’ by insiders. The five-door will stand around 60mm taller than a 5-series Touring, and the look is said to blend sports estate with coupé. Expect a dramatically tapering roofline, and big wheels up to 21in in diameter. In the nose will be six- and eight-cylinder petrol and diesel units, sending power to the rear wheels.

Four-wheel drive – standard on the SUV-based R-class – will be an option in certain markets.

Eschewing standard 4x4 and sharing a car platform means that the super-Touring should weigh up to 200kg less than an R-class. Sounds the ideal formula for producing the ultimate driving MPV. And BMW will have to, to avoid a slow start, R-class-style.