Pressure is growing on the UK government to ratify the Paris climate change deal immediately.
A spokesman for the prime minister told the BBC the UK would ratify "as soon as possible", but did not suggest a date.
But Labour, the Lib Dems, SNP and the Greens say the UK has lost its long-term leadership on climate after the US and China jointly ratified the deal at the weekend.
They say there is no good reason for the UK to delay.
Labour warns that unless ministers start the ratification process right away they will attack the government's "failures" on climate policy in an opposition day Commons debate on Wednesday when a motion will call for immediate action.
It is supported by the Lib Dems, SNP and the Greens.
Some analysts believe ministers are delaying the process until they have their long-awaited low carbon plan in place.
This plan was due last year but postponed until late this year following the chancellor's sweeping cuts to subsidies for clean energy.
The Commons environmental audit committee last week warned that plans for expanding the use of electric cars were failing.
The government has not yet replaced its failed Green Deal scheme to insulate homes - a problem green groups say ministers appear unable to solve.
The opposition parties say they want a return to the long-term cross-party consensus on climate policy, which they say was ended by former chancellor George Osborne.
Labour's shadow climate change minister Barry Gardiner says he has offered an "olive branch" to Business Secretary Greg Clark, who has previously expressed great concern for the climate.
Mr Gardiner says he will turn the opposition day debate into a co-operative policy exercise if Mr Clark begins ratification beforehand.
Mr Gardiner told BBC News: "The normal procedure for domestic ratification would see the government issue a Command Paper, which would be discussed by a small committee of MPs.
"The government has not indicated it is ready to do this to allow the UK to be one of the founding parties to the agreement when it comes into force - most likely by the end of this year.
"We need to re-establish common ground on this vital issue. I am holding out an olive branch to the government.
"Their acceptance would be a welcome acknowledgement that climate change is not just one of the most important challenges we face but is one where all politicians must be seen to be working together."
Many observers at the Paris climate conference judged that former prime minister David Cameron's speech, urging collective action on climate change, was one of the finest - and the UK has played a long-term leadership role on climate change.
Liberal Democrat spokeswoman Lynne Featherstone told the BBC: "Now America and even China have ratified the Paris Agreement, Britain must move as swiftly as possible to sign.
"This is a hugely important example of where Britain should be working with our European partners to set an example for the rest of the world."
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