Formal talks between the two parties are due to begin this week after they were announced by First Minister Nicola Sturgeon when she laid out her government priorities in Holyrood. It has also been revealed the Greens could end up with junior ministerial roles if an agreement is reached.
The move comes despite the SNP winning 64 MSPs at the election earlier this month – just one short of an overall majority – while the Scottish Greens, led by Patrick Harvie and Lorna Slater, returned eight MSPs.
However, Alex Neil, who served in both Alex Salmond and Ms Sturgeon’s governments, but stood down as an MSP at the election earlier this month, has now urged the First Minister to be cautious in the coming negotiations.
He said there was no precedent for what was being proposed – despite coalition governments between Labour and the Liberal Democrats in the first two sessions of the Scottish Parliament – and claimed the Scottish Greens had proved unreliable in the past.
“If you are dealing with the Greens, you have to sup with a very long spoon,” he said.
“They are the only party who ever reneged on an agreement in my experience and that was Patrick Harvie in the budget talks in 2009.
"We only discovered that he was not going to back our budget just minutes before the vote. So there is a big question mark over the reliability of the Greens in my mind, and in the minds of many in the SNP, and if a deal is struck then Nicola Sturgeon needs to be certain why she is doing it, what she wants out of it and how the agreement will be written to ensure the Greens stick to it.”
Mr Neil was a Cabinet minister in 2009 when John Swinney’s first attempt to get his budget passed that year ended in a shock defeat after the two Green MSPs failed to support it at the 11th hour.
Despite a government offer of a £30 million home energy efficiency scheme, Mr Harvie said the initiative fell short of his party's proposed £100m-a-year free home insulation scheme.
The budget was passed at the second attempt, after Labour and the Lib Dems won concessions in return for their support, but the two Green MSPs still refused to back it.
In the past five years, however, the Scottish Greens have supported SNP budgets.
Mr Neil said he understood the “attraction” of a formal agreement “making life easier”.
But he added: "There’s no precedent for this arrangement in the respect that in 1999 and 2003 Labour was very short of a majority, so there was a very compelling case for coalition with the Liberal Democrats.
"When the SNP was a minority government in 2007 with just 47 MSPs there was talk of an agreement with the Liberal Democrats, but the sides couldn’t agree so it didn’t happen.
“But none of that is not the case now given the SNP has 64 MSPs and I am not sure why Nicola Sturgeon is pursuing an agreement of any kind with the Greens. The SNP has the votes and it would be difficult to imagine any situation where it couldn’t get at least one more vote to get measures through.
“On independence, they are obviously going to support a referendum bill when she brings it forward.
"However, the big issue is what do they do when the Tories and Boris Johnson stick to their guns and continue to say no to a section 30 order. Having a coalition or partnership doesn’t deal with that.
"So there needs to be a contingency plan to deal with the Westminster veto in the agreement. The SNP need to have the Greens on the same page.
"Of course, it’s a democratic outrage to refuse a referendum, but the Tories have just been returned with their best ever vote in a Scottish Parliament election on a hardline, anti-independence platform.
"At some point there has to be a plan agreed as to what to do. They can’t just take no for an answer. People who support independence will be looking for a plan in principle for a strategy in this agreement.”
UK Government sources told The Scotsman any deal between the two parties would not shift its position on the granting of a section 30 order, which would give Holyrood the go-ahead for a second referendum.
"Just because the Scottish Government might suddenly include Patrick Harvie in its team of ministers really doesn’t affect us,” one source said. "Why should we change our minds because the Greens are being included? It makes no difference to us.”
A second source said: “We will continue to focus on the recovery from the pandemic and we would encourage the Scottish Government to do the same, with or without the help of the Scottish Greens.”
Asked about discussions on independence with the SNP, Scottish Green MSP Ross Greer said that even if an agreement was not reached during formal talks, the pro-independence majority in the parliament would ensure any referendum bill was passed.
He said: “We will want to talk about independence and all the issues around a referendum, but we won’t need to negotiate as such because we’re not coming at it from two different positions. We will talk about our approach to that [a referendum] and whether we want a joint approach.
“And we’re certainly keen to take the Yes movement with us, but this is not an agreement for independence. It’s an agreement about what we can do right now with the powers that we have in terms of the Covid recovery and climate crisis. Those are the big priorities.”
Mr Greer added: “I’m not trying to downplay the importance of independence – it’s the opposite of that. We already agree, it’s a shared mission.
"The manifestos were close to identical and we all know we’re committed to a referendum in this term of Parliament.”