A prototype of the next-generation Smart Fortwo has been spied undergoing testing in Europe.

The test mule in these spy shots wears a rough-cut combination of new panels and existing bodywork from the current Renault Twingo, emphasising the French brand’s role in the joint venture with Smart’s parent company Daimler that will give life to at least three all-new models.

The patchwork exterior gives us little insight into the design of the upcoming Smart Fortwo, which is expected to draw inspiration from the Smart For-us baby ute concept that debuted at the 2012 Detroit motor show.

More important is the prototype’s rear-wheel drive architecture, which is set to underpin the third-generation Fortwo in a short-wheelbase form and stretch to accommodate the larger bodies of the next Twingo as well as a reborn version of the Smart Forfour small car.

The all-new Smart Fortwo’s will be available with a choice of low-capacity petrol and diesel engines as well as a zero-emission electric powertrain.

All three new models are set to launch around 2014, with the Fortwo rumoured to make its debut at the 2013 Frankfurt motor show next September.

Dubbed ‘Project Edison’, the city car collaboration is part of a wider partnership between Daimler and Renault involving a number of the groups’ mainstream brands – including Mercedes-Benz, Nissan and Infiniti – that is estimated to save both parties two billion euro ($2.4 billion) over the next five years.