No more than one third of carbon-offset companies meet the requirements of the association recently set up to codify standards in the sector.

That is the view of Jonathan Shopley, executive director of offset-provider Carbon Neutral and co-chairman of the International Carbon Reduction and Offset Alliance.

ICROA was launched in June with eight founding members which offer offsets to air travellers including through the travel trade.

Shopley estimates 140 companies and non-profit organisations around the world offer carbon offsets in a sector that has exploded in the past decade, leading to accusations of profiteering.

Shopley said: “ICROA was set up to provide standards and make offsetting transparent. Only about a quarter or one third of companies meet the ICROA code now, but we hope to have 30 to 40 organisations as members in a year.”

He said some companies would be excluded by the requirement to have been in business for a year or by the £5,000-a-year fee. But others would fail because the ICROA code excludes “wild west players”, he said.

Shopley believes offsets for air travel could include an allowance for the added effect of flying at altitude – an effect known as radiative forcing – within a year.

Some scientists argue this increases the global-warming impact of aviation by a factor of 2.5. The Department for Transport suggests a factor of 1.9, but the industry disputes this and the science is inconclusive.

Shopley suggests the industry could agree a figure that would double the amount of CO2 offset until the science is clear.

He said: “The science is difficult and we do not have a view as to what the radiative-forcing index should be, but we need to resolve the issue.”

Alice Chapple, a director of sustainable development organisation Forum for the Future, said: “Carbon offsetting needs to retain its credibility to maintain momentum. We need robust standards.”