The Scottish National party will find it far harder to govern unless the next first minister agrees to work with centre-left parties, a co-leader of the Scottish Greens has said.

The Greens were in a power-sharing deal with the SNP until Humza Yousaf unilaterally ended it last week, precipitating his downfall on Monday.

In an interview with the Guardian, Patrick Harvie said Yousaf’s successor as first minister would face “knife-edge vote after knife-edge vote” or be forced to do deals with the anti-independence Conservatives unless they kept the Scottish Greens on side.

He refused to speculate on whether the Greens favoured John Swinney or Kate Forbes of the SNP leadership frontrunners, but his remarks will be widely seen as a warning to Forbes and her supporters.

Forbes is a social conservative opposed to many policies championed by the Greens around gender identity, abortion clinic buffer zones and climate action. The Greens have repeatedly said they are very unlikely to support her as first minister.

“Whoever the SNP choose is still going to have to command a majority in parliament, they are still going to have to reach out,” he said. “There is a clear consensus across Scottish politics for a broadly left-of-centre economic agenda which has fairer taxation, redistributive policies like the Scottish child payment, and does what it can to stand up to austerity.”

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