Charles got behind the wheel of the lightweight Rasa when he visited eco-vehicle company Riversimple in Llandrindod Wells, Mid Wales, yesterday. Taking the two-seater for a test drive, Charles heard how the Rasa emits nothing but pure water vapour. It is also designed to minimise the pollution from tyres and brakes with its low weight, skinny tyres and efficient braking.

       It has a range of 300 miles and is billed as being “light to handle, responsive and fun to drive” with a “nippy 0-60mph in 9.7 seconds”.

       The prince, known for his green credentials, was pictured grinning as he emerged from the aerodynamic car through its hi-tech butterfly doors on his annual summer week tour of Wales.

       Hydrogen offers a green transport alternative because all that comes out of the exhaust is water.

       But hydrogen is only as green as the method by which it is produced. “Blue hydrogen” involves splitting it from natural gas which requires a lot of energy and makes the greenhouse gas carbon dioxide.

       “Green hydrogen” uses electrolysis powered by renewable energy to split hydrogen from water. But this process, like blue hydrogen, is currently expensive.

       Supporters hope that if the industry takes off the price will come down through economies of scale.

       Riversimple is building Rasas for trials with the general public, both in Monmouthshire, where they have installed a refueller in the town centre of Abergavenny, and in Milford Haven in Pembrokeshire.

       It is the first in a range of vehicles, including a light commercial van and a family vehicle, that the firm plans to offer to customers for hire.