IT sounds like a secret weapon out of a sci-fi movie but the e-tron is the most technically sophisticated Audi to hit the road.

As a plug-in hybrid this version of the award-winning Audi A3 uses electricity to boost a 1.4-litre turbocharged petrol engine resulting in theoretical fuel economy of 176 miles to the gallon.

That's a fanciful figure and will never be achieved in the real world but it's what the e-tron managed in official bench tests and results in ultra low emissions of 37g/km putting it out of the reaches of congestion charging and into the five per cent tax band for business users.

It is a tantalising proposition for motorists who need to keep costs down and is one of just a handful of plug-in hybrids available at the moment.

They are vehicles designed to maximise the benefits of zero emission electric motoring without the problem of limited range and the e-tron slots in alongside plug-in versions of the Toyota Prius, the Mitsubishi Outlander SUV, Volvo's V60 estate and the Porsche Panamera.

Audi's offering is in the family-sized A3 Sportback and with the Government's £5,000 subsidy for electric vehicles is priced from £29,995 - £3,500 more than a regular A3 in similar trim.

To drive there is virtually no difference between the new hybrid and a conventional A3 although boot space at 280 litres is about 20 per cent smaller.

That's caused by the size of the lithium-ion battery pack which is slung beneath the back of the car and luggage room will be compromised further by the bag containing the charging cables.

It's also about 300kg heavier than a non-hybrid 1.4-litre A3 and both are front wheel drive.

Inside there are slight changes to the instrumentation to reflect the electrical side of the operation - the rev counter is replaced with a power meter for example - and on the outside the differences are equally subtle with discreet e-tron badges and some aerodynamic tweaks to improve efficiency in the shape of a rear diffuser, streamlined bumpers and a solid panel for the Audi rings on the grille, behind which is the charging socket.

On start up the car comes to life silently in electric mode and as such it has a claimed range of 31 miles at speeds of up to 80mph.

There are three other drive modes available at the touch of a button - automatic, where the petrol engine and electric motor work together, a ‘hold' mode where it is petrol only to conserver battery power en route enabling EV operation to be switched in when required and a regenerative mode which uses power from the engine to recharge the batteries on the move.

There is little difference to the feel of the car in any of the hybrid settings or even when electric only where it still functions with the six speed semi-automatic gearbox, although with slightly more pick up since EV power delivery is instant.

The e-tron has a good turn of speed and is quicker than a conventional 1.4-litre A3 at 7.6 seconds 0 to 60. Top speed is the same at 137mph.

The combined power output from the 75kw electric motor and the 150ps engine is the equivalent of 204ps and that is significantly more than in any other A3.

Where the e-tron will score is in everyday use, completing the average commute driven purely by electricity and on a single charge which costs just over £1 and takes four hours from a domestic supply - half that from a special charger.

Audi claims that the e-tron has a range of 580 miles and on our drive the car showed considerable potential for good economy.

We started off with the instruments showing 290 miles available from the fuel tank and 20 miles worth of electricity.

After a 160 mile journey that included motorway, country and town driving (where we completed nine miles in EV mode) the e-tron was still showing an available range of 210 miles - an overall average of 43.1mpg which is more than to be expected from a non-hybrid A3 but not as much as you would get from a diesel version.

Fuel efficiency with the e-tron is something of a moveable feast and as well as normal considerations such as traffic and speed you also have to factor in weather conditions and the optimal driving modes for the journey - below -10C EV operation is not possible.

Fuel consumption will also be greater in regenerative mode, although at one point in our drive we managed to add another nine miles worth of range into the batteries, restoring capacity to its original charge level.

Audi has proved its e-tron technology works by winning Le Mans three times with hybrid racing cars and compared to battery only models there are many plus points for plug-in hybrids.

A battery car with a range of around 100 miles such as the Nissan Leaf or the electric versions of the Kia Soul and Ford Focus will cost around £25,000 - for another £5,000 you can now get an Audi that can be emission free but will never make you fret about running out of juice.

It's only real rival is the BMW i3 range extender but even that cannot match the A3 for range.

While not the answer to every driver's demands this new generation of car will satisfy most and can slash running costs as well as giving the driver the added street cred of being seen to be green.