SCHEMES USED by some of the world's top companies to offset pollution are flawed and failing to save the planet, it has been claimed.

Oil giant BP has also been accused of exaggerating cuts in emissions from pig farms in Mexico, while Sky TV's claims for a renewable energy plant in Bulgaria may prove illusory.

Estimates used by British Airways to offset pollution from air travel were also described as being too low. And trees planted in Britain to reduce greenhouse gas emissions may prove ineffective.

These revelations, to be broadcast in a Channel Four Dispatches documentary tomorrow evening, will re-ignite fears that much of the burgeoning carbon-offsetting business could be a con.

"Carbon offsetting is more about salving one's conscience than solving the problem of climate change," said Duncan McLaren, chief executive of Friends of the Earth Scotland, warning that "carbon cowboys" could damage real efforts to combat global warming.

Many companies are claiming green credentials by reducing their carbon footprint or aiming to become carbon-neutral. But some of their projects do not live up to expectations.

Until recently BP said on its website that trapping methane emissions from hundreds of pig farms in Mexico could cancel out pollution from three-quarters of a million cars. But the methane savings are turning out to be only about half the predicted levels.

BP confessed that it has bought only enough offsets to account for 2500 cars. It admits anticipated saving from pig farms "will not be reached" and has corrected the claims on its website.

Sky television has offset 12% of its pollution by paying The Carbon Neutral Company (TCNC) to invest in a hydro-electric plant at Katuntsi in southwest Bulgaria. In an interview with Dispatches, the plant manager, Vladislav Tsvetkov, said the money was "not required" to make the business work. Later, however, he insisted the investment was "critical" and TCNC said that it "increased the financial performance of the project". But the bank in Sofia that financed the plant said that the carbon money "was not a prerequisite".

Sky said it had had assurances from TCNC about the investment. "We regularly review our approach and if it is shown that any offsets do not meet their claimed environmental contribution they will be replaced," said a Sky spokesman.

TCNC argued that offsetting had a vital role to play in cutting pollution. "We work with 150 projects and demonstrate, through independent monitoring, that they reduce greenhouse gas emissions," said a spokesman.

Another UK carbon company, Climate Care, has a deal with British Airways for offsetting emissions from flights. But estimates of the impact of pollution, and how much to pay for it, vary.

Climate Care itself quotes £39.79 to offset emissions from a return flight from London to Adelaide, Australia. But BA's online emissions calculator, provided by Climate Care, says offsetting the same flight will only cost £27.58.

The problem is that BA does not accept that emissions from planes are as damaging as experts say. Climate Care's founder, Mike Mason, said he wished BA would change its mind.

TCNC is also involved in financing tree-planting projects to offset pollution. Among its many clients are Honda UK, Avis Europe, and Barclays Bank. But there are doubts about whether the forestry projects really deliver. The owner of Donkleywood in Northumberland, Bryn Howson, said TCNC's investment hadn't been "absolutely essential" for the creation of the woodland. He received £55,000 from TCNC for the right to sell 20,000 tonnes of carbon dioxide offsets, but most of the funding, £320,000, came from the government's Forestry Commission. TCNC argued that the money it provided was necessary as the "last piece of funding".

Dr Richard Dixon, director of WWF Scotland, accepted that many renewable and forestry offsetting projects did useful things. "But these ones look like they would have happened anyway," he argued. "Companies are paying to appear green and encouraging their customers to think they have done their bit, when in fact little or no real contribution has been made to cutting emissions."

Dispatches: The Great Green Smoke Screen Channel Four, 8pm, tomorrow