The world must reach an agreement to cap aviation emissions at UN climate talks in Copenhagen in December if countries are to meet targets to combat global warming, Britain's chief climate adviser said yesterday.

The Committee on Climate Change, which advises the British government on carbon reduction, said a global agreement must bring carbon dioxide emissions from developed nations' air travel at or below 2005 levels by 2050.

"It is vital that an agreement capping global aviation emissions is part of a Copenhagen deal," said committee chief executive officer David Kennedy in interim advice to the government.

"We are calling for a cap that would not require people to fly less than today, but would constrain aviation emissions growth."

Failure to take urgent action could lead to flights producing up to a fifth of all global carbon dioxide emissions by 2050, from about two per cent now, the committee said.

The EU has already agreed to a five per cent reduction in net aviation emissions from 2013-2020 and any global deal should at least match that, Kennedy added.

Foreign Secretary David Mili-band said on Tuesday there was a real risk the Copenhagen talks will fail as politicians focus on the economic downturn rather than the longer-term threat of climate change.

Radical changes to planes' engines, fuel and airframes will play an important part in any plan to cut emissions, the committee said. Countries must identify ways of helping the aviation industry to fund extra research in this area.

Governments should also plan for an interim period where aviation emissions can be offset by reductions in other sectors using cap-and-trade markets, the committee said.

However, in the longer term "the vast majority" of cuts must be made without the purchase of carbon credits, it added.

The committee will publish fuller advice on how countries will be able to reduce aviation emissions in a report on December 8.

In a joint statement, Britain's energy and transport departments said, "We will take full account of the committee's advice as we continue to press for international aviation to tackle climate change at Copenhagen".