The Prince of Wales’s private income rose by 7 per cent to more than £16 million last year, yet his tax fell by £5,000 to £3.4 million, Clarence House accounts showed today.

The money generated by the Duchy of Cornwall - the landed estate given to the heir to the throne - increased by just over £1 million to £16,273,000 during the last financial year.

The accounts also showed that Charles’s official spending, including money spent on charitable work and official duties, was £10.4 million pounds. But his personal costs, which includes the salaries of 30 full-time staff, fell from £2.6 million to £2.2 million.

However the prince's thrifty nature appears to have skipped a generation. It was revealed today that five controversial helicopter flights Prince William made while training with the RAF cost more than £50,000.

The trips, which included journeys to a stag-do on the Isle of Wight and the family home of William’s girlfriend, Kate Middleton, were criticised as an alleged abuse of military training. The Ministry of Defence acknowledged there had been “a degree of naivety in the planning of the training sorties”.

The accounts also revealed that while Charles's income swelled, his carbon emissions plunged. The amount of carbon dioxide produced by Prince Charles fell by 18 per cent to 2,795 tonnes during 2007-08, the accounts revealed. The drop means he is on course to surpass his target of reducing his greenhouse gases by 25 percent between 2007 and 2012.

Clarence House officials said the cut was due to a switch to “green” electricity supplies and a further reduction in travel-related emissions. The Prince has lived up to his promise to take fewer plane journeys, spending 22 per cent less on official travel by air and rail this year, a drop from £1.4 million to £1.1 million.

The heir to the throne, renowned for his keen interest in all things environmental, has also reduced his motoring emissions by running his 38-year-old Aston Martin on bio-ethanol made entirely from English wine.

"The bio-ethanol from our supplier happens to be bio-ethanol made from wine," Michael Peat, his private secretary, said. "I think our wine is surplus English wine. It is wonderful. It is not corked."

His other cars - a Jaguar, an Audi and a Range Rover - run on biofuel made from cooking oil. The royal train, which Charles and Camilla use to travel to certain engagements, has also been converted to run on biofuel.

The eco-friendly prince has installed wood-chip stoves at his country homes - the Highgrove estate, where he farms organically, and Birkhall in Scotland - and energy-efficient boilers at Clarence House.

The Royal carbon footprint would have shrunk further had it not been for an increase in emissions from the prince's farm, which has increased its animal stocks this year.

But Mr Peat explained that proportionally, the farm released fewer harmful emissions than others due to the less gaseous diets of its animals. "We think our animals release less methane because they eat grass and not as much grain or feed." he said.

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