Former Liberal Democrat leader Charles Kennedy has poured cold water on an offer from the Conservatives to forge a "progressive alliance" against Labour.
He said there were too many issues dividing the parties.
Tory leader David Cameron had suggested that he wanted to work with the Lib Dems to pressure Labour into decentralising power.
Lib Dem acting leader Vince Cable also rejected the offer, saying Mr Cameron was living "in cloud cuckoo land".
Writing on his personal website, Mr Cameron called on both the Green Party and the Lib Dems to work with his party.
He wrote: "I've always believed that political parties, even though they may have serious disagreements over many aspects of policy, should work together in areas where they agree.
"I hope that in 2008 the Liberal Democrats and the Green Party will join us in putting pressure on the government to decentralise power, and that together we can create a new progressive alliance to decentralise British politics."
But Mr Kennedy said that, although he would not reject outright the idea of working with the Conservatives on specific issues, a longer-term alliance was unlikely.
"I don't see a great consensus between the leaders of the Tories and Liberal Democrats on Europe, for instance, " he told the BBC's AM programme.
"We are in favour of the new EU treaty - they are against it."
Mr Cable said: "A Conservative party that wants to cut funding on public services and talks the talk on green issues but fails to deliver any meaningful policies has little to offer today's Britain.
"Only recently the Tories have revealed their true colours on environmental issues by supporting nuclear power and backing the expansion of major airports."
David Cameron's offer of an pact has been spurned
Mr Cameron's offer appeared to be targeted at both Nick Clegg and Chris Huhne, the two contenders for the Lib Dem leadership succession. The leadership result is expected to be announced on Tuesday.
The offer follows Gordon Brown's earlier attempt to woo the Lib Dems, offering Cabinet seats in his "government of all the talents" to senior party figures, like ex-leader Lord Ashdown.
This was swiftly rejected by the then leader, Sir Menzies Campbell, and caused deep anger within the Lib Dem ranks. It was seen as a deliberate attempt to destabilise Sir Menzies' leadership.