Ministers could reverse their commitment by cutting carbon emissions to match the EU, Chris Huhne has told MPs.
In an announcement viewed by green campaigners as a key test of the coalition's credentials on the environment, the energy and climate change secretary told MPs progress would be reviewed before the end of the parliament.
Mr Huhne said it may be necessary to make changes if the EU's emissions trading scheme was "not ambitious enough".
If UK businesses faced a "disproportionate strain" as a result the government could reconsider in early 2014.
"If at that point our domestic commitments place us on a different trajectory, we will as appropriate revise our budget to align it with the actual EU trajectory," Mr Huhne told MPs.
Campaigners had called for the government to rule out watering down the proposals further by not including carbon trading in the commitment.
But Mr Huhne told the Commons that domestic emissions would only be reduced "as far as is practicable and affordable".
"We also intend to keep our carbon trading options open, to maintain maximum flexibility and minimise costs in the medium term," he added.
"Given the uncertainty ahead, this is a pragmatic approach."
Mr Huhne was revealing to MPs the government's carbon budget for the period from 2023 to 2027.
He said the UK would achieve a 50% reduction in carbon emissions based on 1990 levels by the end of that five-year period.
This would not impose any additional costs in this parliament, Mr Huhne said. The government will bring forward a package of measures by end of this year to help energy-intensive industries adjust.
The announcement followed a week of speculation as to whether the coalition would accept the recommendations of the independent Committee on Climate Change in full.
A leaked letter from business secretary Vince Cable appeared to place his department at odds with that of the energy and climate change secretary, prompting a reported intervention from prime minister David Cameron.
Intensive discussions were understood to be continuing throughout the weekend.
"With all government policy, particularly policies that become bills and then law, there is discussion across Whitehall," the prime minister's spokesman said this morning.
"Different departments have input into that debate. But there is no question it is the government's policy to have a 0.7% GNI target - and we will introduce a bill in due course."