Pumping furiously on a foot treadle in the afternoon heat, six-year-old Sarju Ram is irrigating her impoverished family’s field, improving the crop and – without knowing it – helping environmentally sensitive holiday-makers assuage their guilt over long-haul flights to dream destinations.

But Sarju and her four brothers and sisters working flat out in a clump of trees that provide scant shelter from the sun illustrate a growing argument over claims that British environmentalists’ efforts to curb greenhouse emissions are inadvertently fuelling an increase in child labour.

Sarju’s family is a beneficiary of Climate Care, an organisation that helps some of Britain’s leading public figures and companies to offset their carbon dioxide emissions by funding sustainable energy projects.

The Prince of Wales turned to Climate Care after his environmental adviser, Jonathon Porritt, worked out the prince’s carbon footprint.

Customers of British Airways are among those who have been encouraged to log on to Climate Care’s website and calculate how many tonnes of greenhouse gases their flights will generate, and how much it will cost to neutralise the impact on the atmosphere. A flight to Barbados for a family of four, for example, generates 7.55 tonnes of carbon dioxide, which will cost them £56.64 to offset.

Climate Care uses the money to help persuade families such as Sarju’s to give up labour-saving diesel pumps and buy human-powered treadles instead. It claims that by using the treadle, a family will save money on diesel and hire charges, earn more from increased crops and cut the carbon emissions that would have been produced by the pump.

Last week Indian experts criticised the scheme, saying it was promoting child labour and forcing poor farmers to work harder so that wealthy air travellers could enjoy exotic holidays without worrying about the environment.

“The problem is the number of times child labour is involved,” claimed Ashutosh Pandey of Emergent Ventures India, which advises companies on clean technology.

“It’s not being monitored properly. It’s not reducing emissions. People are selling their diesel pumps to others who are using them.”

Sunita Narain, an environmentalist at the Centre for Science and Environment, a Delhi think tank, said: “It won’t help global warming if people take more flights to the Seychelles.”

Michael Buick, a spokesman for the Oxford-based Climate Care, confirmed that children were working the pumps it promotes, but said that people had to focus on the benefits to the whole family.

He said his group was proud of its scheme, which had led to more than half a million foot treadles being sold, and had won several awards. Four reports had identified major benefits.

According to Buick, critics are mistaken in claiming that diesel pumps are better than human-powered alternatives, because they are costly to run. The treadles meant farmers could rely on increased crops.

Buick said that by “all mucking in” families were able to increase yields and earn more to pay for children to go to school. The extra income also meant fathers could stay with their families rather than leaving them to look for work in the cities.

“If mum is planting and harvesting, the daughters help out. It’s just a different way of life. The phrase ‘child labour’ is emotive. It implies factories, but these are family farms where everyone gets stuck in, watering the crops and taking a turn on the treadle pump,” he said.

Last week in Pairapur village, Sarju and her family took turns on the pump, 30 minutes for each child and an hour for each parent. They are growing more corn, aubergines, and papayas, they said.

They had been paying £1 an hour to hire a diesel pump, said Sarju’s father Hari Ram, and had fallen into debt. They have now repaid a £900 loan and for the first time are saving money. “Our life has improved a lot,” he added.

But Sunita Narain said the rich needed to cut their own emissions by 50% rather than relying on offsets. “An increase in manual labour is an ethical issue,” she said.

More info about the the manual water pump - and watch a video of the pump being used by an Indian farmer in Utter Pradesh