The designers have done...

The designers have done little to the Z4"s shape other than add a sleek metal roof complete with large rear windows.

As with its predecessor - based on the Z3 - the newcomer will be aimed at buyers seeking a top driving experience. Precise details of the model line-up are still secret, but BMW is expected to offer a new 3.2-litre six-cylinder variant producing 335bhp. That will sit alongside an entry-level 268bhp 3.0-litre car, using the powerplant from the roadster.

The range is set to be launched across Europe in autumn 2006, and the flagship will wear the M badge reserved for the firm"s most extreme models. Insiders have hinted that engineers wanted to use the V8 from the next-generation M3, but it wouldn"t fit in the Z4"s engine bay. The fixed-roof car will be joined by a new 3.2-litre version of the roadster.

Before the introduction of the hard-top, BMW will facelift the current Z4 range. Due to be unveiled at the Frankfurt Motor Show in September, the revised two-seater will get new bumpers front and rear, complete with fresh light clusters. There will be several interior changes, too, bringing quality into line with that of the latest 3-Series. It"s also thought BMW bosses will introduce an iDrive-style cabin control system.

Meanwhile, designers at the firm are already looking further ahead - to the next-generation Z4. Codenamed E89, the replacement roadster isn"t due until 2009, but will showcase a new look while retaining the current distinctive shape. It"s set to be bigger than the existing car, and the bodywork will have to be altered to comply with forthcoming pedestrian impact regulations.

While no official decision has been made, top brass are deliberating over whether to fit a folding hard-top or a conventional fabric hood. A metal roof gives greater insulation and refinement, but forces stylists to pen a large, flat rear deck to hide the panels and motors.