The motor industry’s worst kept secret is finally out in the open: Porsche will launch a fourth volume model line – a ‘premium class Sports Coupé’ – in 2009.

Chairman Wendelin Wiedeking confirmed the green light for the Grand Tourer, named Panamera, in late July. “We have indeed taken a long time in making this decision. But now we know that the Panamera is the right car for Porsche: its performance, design and driving dynamics will all meet our high standards,” he said.

With four years to go to launch, official details are scant. Wiedeking confirmed that the Panamera will have four doors and four seats, atop a chassis that mounts its engine at the front, driving the rear wheels.

There was one big revelation, however: the Panamera will have a bespoke, Porsche-engineered platform.

The Stuttgart grapevine has long rustled with whispers that Panamera R&D would be shared between VW and Porsche, as with the Touareg/Cayenne. VW could have used the platform for a Phaeton MkII and the long-rumoured C1 project, a high roof hatchback in the mould of Vauxhall’s Signum. But it was not to be.

Eliminating a link with another manufacturer makes imported engines, such as the Cayenne’s VW-sourced V6, a long shot. Expect the Panamera to get the latest Porsche powerplants, such as a petrol V8, which has to be overhauled to meet emissions regulations. In naturally-aspirated guise, the V8 should make 360bhp; add a couple of turbos and peak output should pass 500bhp. A revised version of the Carrera GT’s V10, pumping out more than 600bhp, should power the flagship version.

Like the 911, there should be four-wheel drive versions, but the Panamera doesn’t merely look like a four-door version of Porsche’s flagship. The nose is long and sleek, with the engines pushed back behind the rear axle for optimum handling. The lights are said to resemble the Carrera GT’s, the front air scoops gape, á la 911 Turbo. As for those four doors, all are front-hinged, eliminating any Mazda RX-8-style showroom squeals of delight.

Porsche predicts 20,000 sales a year, but if the success of the Cayenne is anything to go by, that target looks very modest indeed.