The government has committed to an 80% cut in carbon emissions by 2050
The government is fighting to head off a backbench rebellion over its plans to exclude aviation and shipping from the UK's greenhouse gas targets.
They are being left out because there is no system for sharing responsibility for international emissions.
Fifty-six Labour MPs are demanding the sectors be included, enough to defeat Gordon Brown when the Climate Change Bill goes to a Commons' vote next week.
Campaigners say it is "unfair" to give the sectors "special treatment".
Friends of the Earth said a climate change law which left out emissions from planes and ships was like "a drink-driving law that doesn't count whisky".
Last week, Climate Change and Energy Secretary Ed Miliband announced a government commitment to an 80% cut in carbon emissions by 2050.
But that did not include emissions from international flights and shipping after the government-commissioned Turner Report said it would be too difficult to share out responsibility for the gases they produce between different countries.
Rebel MPs now say they want that decision overturned.
They want an amendment to the Climate Change Bill to state that if emissions from aviation and shipping continue to grow, the government must compensate with extra CO2 cuts elsewhere.
This law is a world first - we now need to make sure it's world-class
Nigel Griffiths MP
BBC environment analyst Roger Harrabin said he understood ministers may be about to concede that principle, but will insist there is still uncertainty about how to account for international emissions.
The amendment was tabled by Edinburgh South MP Nigel Griffiths, who said he was "very encouraged" by discussions he has had with Climate Change Minister Joan Ruddock.
"Addressing issues of aviation and marine shipping emissions is now the acid test of the government's aim to achieve a genuine reduction in CO2 emissions," he said.
"This law is a world first - we now need to make sure it's world-class."
Friends of the Earth executive director Andy Atkins said: "Giving special treatment to the shipping and aviation industries is unfair and doesn't make sense.
"Ed Miliband has promised he will deal with emissions from planes and ships, but voluntary commitments are not enough.
"This pledge needs to be set in law - only then will the public have confidence that this and future governments are going to deliver."