The European Union on Tuesday (4 March) reached a preliminary deal on a law that will exempt long-haul flights from paying for carbon emissions until 2016, EU sources said.
The deal is a further weakening of the bloc's stance following immense international pressure and threats of a trade war.
The sources said negotiators from the European Parliament, the European Commission, and the EU presidency, representing member states, had tentatively agreed that an existing suspension of EU law for intercontinental flights should be extended.
A meeting of EU diplomats representing member states is expected to debate the deal and possibly endorse it on Friday.
A spokesman for the European Commission had no immediate comment.
Environmentalists were swift to criticise Tuesday's outline deal and called on the European Parliament to reject it.
Without parliament's approval, the original law, covering the length of intercontinental flights into EU airports, would reapply, raising the risk of a new outbreak of trade threats.
EU diplomatic sources, speaking on condition of anonymity, said the agreement would maintain a suspension of the law for intercontinental flights until 2016, with a provision to revert back to making all aviation pay for allowances in 2017 if a global deal on curtailing aviation emissions cannot be agreed.
"With today's deal, European governments have conceded again to international pressure without getting anything meaningful in return, let alone guarantees that soaring international aviation emissions will one day be tackled," said Bill Hemmings, aviation manager at campaign group Transport & Environment.
Together with the European Green party, he urged the European Parliament to reject the deal.
"It is reckless to dismantle this effective climate policy instrument in exchange for a vague promise on a global scheme in the distant future," said Satu Hassi, climate change spokeswoman for the Greens.
An EU law on requiring all aviation using EU airports to pay for their emissions by buying allowances on the EU Emissions Trading System led to threats of an international trade war.
Non-EU nations, including the United States, India and China, accused the European Union of breaching rules of sovereignty and threatened retaliation.
Eventually the Commission agreed to suspend the law for intercontinental flights, but on condition a global alternative was drawn up. The law has always remained for intra-EU flights.
The UN's International Civil Aviation Organization (ICAO) in October agreed it would deliver a global plan to curb airline emissions by 2016 for implementation in 2020.
The Commission's response was to propose amended legislation just charging aircraft for emissions in EU airspace, rather than for the entire flight.
But that too prompted international criticism and leading EU members Britain, France and Germany proposed it should be scrapped, paving the way for Tuesday's watered down deal.
- March 2014: Vote in the European Parliament's plenary session