Two of the most important new cars ever are about to ‘bubble’ to the surface – and in a world exclusive head-to-head, Auto Express has brought them together for the first time!

These are the clearest illustrations yet of BMW’s new Isetta and its sister, the Fiat Topolino. They are the result of a new deal between the two firms, which will also be extended to include next-generation versions of the MINI and Alfa Romeo MiTo.

The Isetta and Topolino are crucial to BMW and Fiat’s attempts to reduce the overall CO2 emissions of their ranges – but they will also offer buyers 100mpg fuel returns! Reviving a name not seen since the bubble car of the Fifties, the Isetta will take BMW into a new sector of the motoring market.

It will be joined by the Fiat Topolino – which we featured in Issue 1,013. Both machines will offer incredible fuel consumption and low emissions, as well as tiny dimensions, and rival the imminent VW up! and Toyota iQ.

The Isetta will be hugely important, because it will help BMW comply with the European Commission’s plans to impose an average 120g/km limit on the model ranges of all firms selling cars across the Continent by 2012. And, as there are plans for an electric version, it would also enable the manufacturer to meet California’s upcoming zero-emissions vehicle legislation.

As you can see from our artist’s impression, the Isetta harks back to the original bubble car shape. But it has been given a modern reworking by the design team, with the addition of some BMW-style chunkiness. The body is made up of simple panels, likely to be fashioned from plastic to save weight and help resist parking dings.

Meanwhile, the Topolino gets a retro look inspired by the 500, with a cheeky front end featuring a smooth nose and large headlights. Under the skin of both models lies an all-new platform that places the engine in the rear and sends drive to the back wheels.

With an electric-only Isetta some way off, small-capacity petrol and diesel engines will feature initially. And as Fiat is using a new 900cc two-cylinder petrol motor in the Topolino, the Isetta could benefit from the same unit.

The new powerplant was previewed in Fiat’s Panda Aria concept at last September’s Frankfurt Motor Show, and it arrives in dealers under the bonnet of the 500 later this year. While two outputs will be offered in this car – a 105bhp turbo and a normally aspirated 65bhp unit – it will be the latter that powers the Topolino.

Twin-clutch semi-automatic gearboxes are likely to be used by both models, as is a stop-start set-up which switches off the engine when the car is stationary in heavy traffic. Combine this technology with a lightweight body, and the newcomers will be incredibly efficient. Expect combined fuel economy of 100mpg and CO2 emissions of less than 90g/km.

The interior layout is still up for debate, but there is likely to be enough room for three occupants with an occasional fourth seat, allowing owners to juggle luggage capacity with passenger space.

It will be interesting to see if BMW and Fiat can make the rear-engined, rear-wheel-drive layout work efficiently. An insider at VW recently told Auto Express that a similar mechanical layout in the up! is being ditched in favour of a front-engined, front-wheel drive format, after it was found that the layout was too costly to develop and made the car unstable in cross winds.

Although official production dates have yet to be decided, the Isetta and Topolino are set to arrive some time in 2010.