The government published a ground-breaking climate change bill on Thursday, starting a parliamentary process that could lead to a legal limit on national emissions of carbon gases within six months.
The bill sets a target of cutting national emissions of climate-warming carbon dioxide by 60 percent by 2050.
It would make Britain the first country to adopt such a legally binding commitment.
"This bill is a landmark in environmental legislation and will set us firmly on the path to the low-carbon economy we know is fundamental to our future," said Environment Secretary Hilary Benn.
"This bill shows the world that we're serious and that we're not asking other countries, and in particular poorer countries, to do what we're not willing to do ourselves."
Environmentalists and many politicians had campaigned for a higher goal of 80 percent and annual targets along the way.
But the government has rejected annual targets in favour of rolling five-year "carbon budgets" and has until recently ruled out raising the end goal above 60 percent.
Benn said last month he would ask a climate-monitoring committee to be set up by the bill to see if 80 percent was necessary or feasible.
But environmentalists point out that there is no sense of urgency in Benn's proposal as the first tasks facing the committee next year will be setting the first three five-year carbon budgets and only then will it look at the end goal.
"The government must strengthen its proposals to make it truly effective," said Tony Juniper, head of the environmentalist group Friends of the Earth.
"This means a cut of at least 80 percent in emissions by 2050, including pollution from Britain's share of international aviation and shipping, and setting annual milestones to ensure that we stay on track," he added.
Scientists say global average temperatures will rise by between 1.8 and 4.0 degrees Celsius (3.2 and 7.2 degrees Fahrenheit) by the end of the century due to burning fossil fuels for power and transport.
This will cause floods, droughts and storms, and threaten millions of lives.
Environmentalists also note that while Britain is on track to meet its Kyoto Protocol commitment to cut carbon emissions by 12.5 percent from 1990 levels by 2012, that is more due to the decline of its smokestack industries than good planning.
United Nations environment ministers will meet on the Indonesian island of Bali early next month to try to agree to negotiate a successor to Kyoto which is the only international carbon-curbing treaty but which expires in five years' time.
Reporting by Jeremy Lovell and Tim Castle
Return to green news headlines
View Green News Archive