Sir Menzies indicated co-operation remained a possibility

Sir Menzies Campbell said co-operation required "urgent agreement on specific measures" but claimed the Tories were not able to provide that agreement.

He added that they would be prepared to work with David Cameron when he had some specific ideas to discuss.

The plan to work together had a rocky start in May with the Tories accusing the Lib Dems of playing politics.

Sir Menzies said: "The whole idea of the cross-party approach was to allow all parties to be braver in putting forward new ideas, but the Conservatives are unable to commit.

"Nuclear power, fuel duty, vehicle excise duty and the climate change levy are all areas where we believe we have to make a judgement now."

Off targets?

The issue was brought to a head by attempts to negotiate a joint submission to the all-party climate change group, he added.

Environment spokesman Chris Huhne said: "We could not even get the Conservatives to agree that green taxes should rise as a percentage of national income, even though the leading countries in this field all levy more than we do as a way of changing behaviour.

"It is all very well agreeing to targets, but you have to have some idea of how you are going to meet them if you are going to be taken seriously."

Last month the Conservatives reacted angrily to Lib Dem claims they would use a meeting between the two leaders to test the extent of Mr Cameron's green beliefs.

The Conservatives pointed out it was they who called the meeting between the two leaders, although the Lib Dems say it was in response to a challenge by Sir Menzies during the recent local election campaign.

Since taking over as Tory leader, Mr Cameron has repeatedly raised green issues and appointed environmental campaigner Zac Goldsmith as his adviser.