The European Union's carbon trading scheme may spark a trade war, according to one of the world's biggest planemakers.

"What started out as a solution for the environment has become a source of potential trade conflict," Airbus boss Thomas Enders said.

The Emissions Trading System levies a charge on flights in EU airspace based on carbon emissions.

But the US and China are opposed to their airlines joining the scheme.

The EU scheme, which began on 1 January, creates allowances for carbon emissions and allows airlines to "cap and trade" their allowances.

The number of allowances is reduced over time, so that the total output of carbon from airlines in European airspace falls.

China has banned its airlines from joining the scheme and the US tried to block the introduction of emissions charges late last year, in a case heard by the European Court of Justice.

However, the court ruled that they were legal.