Gordon Brown is facing the prospect of another significant backbench rebellion - this time over climate change.
More than 80 Labour MPs have signed an amendment to the Climate Change Bill, which would force ministers to promise greater cuts in carbon emissions.
The bill commits Britain to make at least a 60% cut in CO2 emissions by 2050. The MPs want that to rise to 80%.
Meanwhile Mr Brown hailed "major progress" at the G8 summit, as leaders agreed to halve CO2 emissions by 2050.
Last year's G8 would only "seriously consider" a 50% cut in C02 but on Tuesday it said it would "consider and adopt" the goal in an international agreement.
Mr Brown said the deal was "beyond what people thought possible". The G8 has agreed a list of 25 areas where wealthier countries can help by cutting energy use, including abandoning traditional light bulbs and reducing power needed by appliances on standby.
The prime minister said he hoped part of that change could see households across the UK switching to electric or less-polluting cars.
This is one of the biggest issues facing domestic politics and clearly goes beyond party politics
Greenpeace said tough targets were needed for the richest countries to slash emissions in the next 100 months.
Mr Brown may face a backbench revolt over Britain's own plans to reduce carbon emissions.
The Labour rebels say the Climate Change Bill's 60% goal will not do enough to control global warming and say the target is based on out-of-date science.
Refusal to change
The government has indicated it is not unsympathetic to the 80% target and the prime minister has already said the evidence points that way.
However, he has handed the issue to an independent Climate Change Committee, headed by former CBI chief Lord Turner, and he is refusing to change the bill until the committee has agreed a new target.
Lord Turner is not due to report until 1 December - after the Climate Change Bill is likely to have passed into law, probably in November.
The rebel MPs want the government to pre-empt Lord Turner's committee and put the 80% goal in the bill now, or persuade the peer to report earlier so that his target can be adopted.
The Climate Change Bill completes its detailed committee stage on Tuesday.
The rebel amendment, backed by 85 Labour MPs - including Tony Lloyd, chairman of the parliamentary Labour Party - was debated a fortnight ago and defeated, as expected, by the government-dominated committee.
But the rebels say their amendment is a marker and are threatening to force a vote on the floor of the Commons at report stage in October or November, if the government does not act.
The Liberal Democrats will also table a new amendment to the bill - calling for an 80% cut - to make sure the issue is debated and voted on when it returns to the Commons after the summer recess.
Environment spokesman Steve Webb said he wanted to ensure the government was committed to the extra cuts before critics within the Treasury and the Department for Business, Enterprise and Regulatory Reform intervened.
"This is one of the biggest issues facing domestic politics and clearly goes beyond party politics," he said.
"We welcome support for the 80% amendment and hope that Adair Turner can produce his advice swiftly so that Parliament has a clear choice in November."
The Conservative frontbench, like the government, is waiting for Lord Turner's report and abstaining on votes on carbon emission targets.
This could save the government from defeat in the autumn.
But there are fears within government that the Tories could harden up their position, back the 80% target and therefore make a government defeat more likely.
Friends of the Earth's parliamentary campaigner, Martyn Williams, said ministers had to "start listening to Parliament and the public and strengthen the Climate Change Bill".
There was "huge support from across the political spectrum for a bill that commits the UK to cutting its emissions by at least 80% by 2050", he added.
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