The Government’s commitment to tackling global warming was mocked last night after it emerged that the long-awaited Bill on climate change had been downgraded.

The measure is to be published next month as a draft Bill for consultation by the public and possibly by a joint committee of both Houses rather than as a fully fledged piece of legislation.

Critics claimed that it meant that David Miliband, the Environment Secretary, had been defeated in his attempt to push through the Bill in this parliamentary session.

Chris Huhne, the Liberal Democrat environment spokesman, said that it augured badly for Mr Miliband’s leadership chances.

But the minister’s aides denied that he had been defeated. He said that it was still his intention to introduce a legislative Bill before the end of the present session to show the Government’s firm intent to go forward on the issue.

An aide said: “It is ridiculous. We have said there will be a Bill this session and there will be. It is not a downgrading.”

Even so, senior ministers admitted that a Bill introduced towards the end of the session would have to be “carried over” into the 2007-08 session if it was to become law. Such a move could only happen with the approval of the new prime minister.

Ministers have delayed the main Bill in the hope of achieving a consensus with the opposition parties over the vexed issue of emission targets. The Government was facing the prospect of defeat in the Commons over its opposition to annual targets for emissions. The Bill would have included a statutory commitment to cut carbon dioxide emissions by 60 per cent by 2050. Many Labour MPs want the party to be bolder and greener in its plans.

Tony Blair and other ministers have argued that annual targets are too restrictive because anything from economic growth to bad weather can cause short-term fluctuations. David Cameron, the Tory leader, has seized on the Government’s difficulties by publishing his own climate change “Bill” proposing the establishment of an independent body responsible for setting annual targets and monitoring to ensure that they are met.

The Bill has been the subject of fierce debate within the Government. The Department of Trade and Industry has argued that British companies should not be put at a disadvantage against foreign competitors, while the Department for Environment, Food and Rural Affairs (Defra) is pushing for Britain to take the lead.

Mr Huhne said: “The climate change Bill will now be presented as merely a draft Bill going for prelegislative scrutiny before a joint committee. The climate change Bill will be postponed to next year at the earliest, and the marine Bill will be even farther down the track.

“This is another example of Defra’s incapacity to push projects through to completion on time, and augurs ill for David Miliband’s leadership credentials.”

The Tories, however, declined to criticise the Government over the decision to bring in a draft Bill. Peter Ainsworth, the Shadow Environment Secretary, said: “I have no problem at all with the decision to go for prelegislative scrutiny. It is important that we get this Bill right and I will not criticise a more consensual approach.”

An argument is understood to be raging behind the scenes over how the Bill should be examined. Ministers favour a committee of both houses and the Tories are believed to favour sending it to the Environmental Audit Committee, chaired by Tim Yeo, a Tory MP.

‘Green paper’

-Puts on legal footing Government’s target to reduce carbon dioxide emissions by 60 per cent from 1990 levels by 2050

-Establishes Carbon Committee-set interim targets, perhaps every five years, but not annually

-Committee to give independent advice on the need to secure energy supplies at competitive prices

-Gives enabling powers for new emission-reduction measures; may include forcing companies and organisations to join carbon-trading schemes

-Monitoring regime and regular report to Parliament on progress towards lower emissions

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