Get ready for yet another surge in popularity for Toyota, as the Japanese company unveils a new, improved Prius hybrid.

Despite packing a larger, more powerful 1.8-litre four-cylinder powerplant, the third generation Prius is expected to boost its mileage from today's 41mpg to a healthy 50mpg.

In fact, Toyota says the larger engine actually helps improve mileage, as the new engine runs at lower average rpm on the motorway.

It also says that mileage is "especially improved in cold-start conditions and at higher speeds" - which we at TechRadar translate as "flooring it to work on winter mornings is actually helping the environment."

Which also means 10 per cent old

The new Prius is 90 per cent newly-developed, shaving a point off its coefficient of drag to make it the most aerodynamic mass market car in the world.

It's the first Prius to eliminate belts from under the bonnet, has a smaller and 20 per cent lighter electric powertrain, and a new braking system with control logic optimised to enhance regeneration.

There is a choice of three driving modes: EV-Drive allows zero emission all-electric driving on battery power alone, although only at low speeds, and only for about a mile, and with an additional caveat from Toyota "if conditions permit." So basically it's for moving the car to a better parking space.

Power Mode increases sensitivity to throttle input for a sportier feel, while Eco Mode, obviously, helps the driver achieve their best mileage.

Solar moonroof raises astronomical questions

Toyota expects to sell 400,000 new Prius cars by 2010, and there are some neat optional extras, including LED running lights, an automatic collision detection system and a moonroof packed with photovoltaic solar cells. These operate a ventilation system that isn't full air-conditioning, but should keep the car cooler when parked on hot days.

Other innovations include touch sensors on the steering wheel that activate different icons on the in-car LCD screen, plant-derived 'eco'-plastics in the seats and trim, radar cruise control and lane keeping systems.

One thing hasn't changed, though. Rear visibility in earlier Prius models has been notoriously poor - the third-gen cars get round this with an optional parking assist video monitor.

The new Prius hasn't got a price yet but should be available, initially in the US, from late spring.