Key Travel, which provides a business travel service designed for not-for-profit organisations, has estimated that charities spending £1m a year on flights would incur offsetting fees of more than £15,000, whereas smaller charities, spending about £230,000 a year, would be billed £2,498.
"In some cases, such as after a disaster, there is no way flights can be minimised, because you can't deal with extreme situations remotely," said Ruth Milner, environment officer at Key Travel. "It's important that charities are able to afford to go abroad if they need to."
December's UN Climate Change Conference is likely to emphasise the need for all flyers to comply with voluntary carbon offsetting schemes.
However, some environmental groups have been dismissive of carbon offsetting as a way of combating climate change. Tony Juniper, director of Friends of the Earth, said: "We need to cut our emissions, but offsetting schemes encourage individuals, businesses and governments to avoid action and carry on polluting."