Top airlines and tour operators keen to flash their green credentials nowadays offer customers carbon "offsets" to compensate holiday pollution.

The problem is that few tourists seem eager to write off their green guilt.

The idea is simple enough: "offsets" are schemes by which a tourist when paying his ticket can also buy into a project elsewhere that will compensate for the amount of carbon dioxide (CO2) resulting from his trip.

But calculating the size of that carbon footprint apparently is not so simple.

A Paris-New York return flight, for example, might emit between one and three tonnes of CO2, depending on the different calculators used by companies and environmental groups.

That means the price of an "offset" can vary by a factor of five, from 15 to 75 euros (25 to 125 dollars).

One of the first to introduce offsets in France, in January last year, was high-end tour operator Voyageurs du Monde. "Voluntary compensations have been a total failure," said company chairman Jean-Francois Rial.

"Only one percent of our clients really paid the cost of the CO2 emitted by his trip," he said.

Now his company simply taxes travellers without first asking their opinion, adding 10 euros to a ticket for a long-haul flight, tantamount to the price for a half-tonne of CO2.

Rial attributed the failure of the system in part to the complications of putting it into place. "Clients have to pay twice," he said, first paying the tour operator for the holiday, then having to agree to pay for the "offset" on an environmental site on the Internet.