Nothing in the automotive industry remains unique for long – and the rivals are lining up to have a pop at the Mercedes-Benz CLS.

Porsche’s forthcoming Panamera, a four-door, four-seat coupé due in 2009, wants a slice of the action. But Porsche isn’t alone, Audi and BMW are also planning to gatecrash the party.

Audi’s contender – likely to be badged A7 – is expected in 2009. To deliver those killer looks, the A7 will share its DNA with the Q5 crossover SUV, due in 2008. Design chief Walter de’Silva will slam down the A7’s swooping body over 18in alloys minimum, creating awesome road presence like a Chrysler 300C’s. It’ll still have four doors and a saloon boot, while a long glass roof will beam light in to the four-seat cabin. There may even be a convertible. Mounted in the nose are V6, V8 and V10 petrol engines; sixpot diesels will also be offered.

BMW is working on a USP – unusual doors – for its competitor. Expect rear-hinged doors, Rolls-Royce Phantom style, to grace the four-door coupé (codenamed F14). The car isn’t due until 2011, which suggests the underpinnings will come from the new big car platform matrix BMW is developing. This will support the 2008 7-series, the next 5-series and the new 6-series.

The engines mirror Audi’s plan, with the 302bhp 3.0-litre twin turbo petrol, a 4.4-litre V8 and an updated version of today’s V10 for the M version. A 300bhp upgrade of the twin-turbo diesel six will figure. Naturally, F14 will be rear-wheel drive, with a six-speed manual or auto, or BMW’s long-rumoured dual-clutch transmission, providing the link.

BMW wants to beat Merc dynamically, so the chassis will get active steering, active anti-roll bars and hi-tech brakes. Just one snag: the MkII CLS will be due around the same time –  and the three-pointed star won’t give up its supremacy without a fight.