The party is keeping its options open over a possible coalition

Members agreed the move after activists had heard that other parties have already begun making informal approaches to the party.

Its annual conference in Edinburgh endorsed plans to study a scheme used by some minor parties overseas.

It would involve a role for the Greens, but only in return for concessions.

They would back the other party in the appointment of a first minister, and would not vote against the first minister's administration in a "no confidence" vote.

They would also support the annual budget, while trying to negotiate spending commitments.

If we have a massive Green vote, then after the next election coalition may well be a possibility Mark Ruskell MSP

The sole pre-condition at present, without which talks with other parties could not even begin, is a veto on any new nuclear power stations.

The talks would then include the Greens trying to push their policy priorities.

Delegates backed the plan at the conference, but also ordered party chiefs to investigate ways in which the Greens could keep a check on the day-to-day operations of an agreement by creating a "reference group" drawn from the wider party.

Mark Ruskell, MSP for Lothians region and the party's campaign director for next year's Holyrood election, told the conference the election offered the party the opportunity of real influence in tackling problems like climate change.

Scrap M74 plan

He said: "What I'm interested in is the Green Party making the maximum impact in government."

Arguing for the Greens' favoured scheme, Mr Ruskell said this had worked successfully in New Zealand where it had created stable minority government.

He explained: "Although we are not excluding other models, we are saying this represents a real opportunity to drive the key demands of the Green Party into the heart of government."

The Greens could demand action on climate change, action to tackle the causes of anti-social behaviour, and other policy priorities, he said.

They could also demand the scrapping of plans for the M74 motorway extension and the Aberdeen western approach road.

"If we have a massive Green vote, then after the next election coalition may well be a possibility," he said.