Scottish Greens sources said that some of the new members had previously been in the SNP and had joined the Greens as they opposed Mr Swinney’s stance on the North Sea.
“We are picking up grumblings from the SNP with people unhappy with the current trajectory with John effectively dropping the presumption against new oil and gas,” said one insider from the Greens.
“They’ve had a number of members quit and we’ve had a bump. We’ve had thousands in new donations since conference.”
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At the SNP’s annual conference in Aberdeen last month, SNP delegates backed a motion including a principle that drilling domestic oil and gas was “clearly preferable” to more environmentally damaging imports.
It marked a retreat from Nicola Sturgeon’s outright opposition to new North Sea drilling, a position some in the SNP regarded as increasingly out of step with public opinion against the backdrop of war in Iran.
However, following the announcement by the UK Government that it would approve drilling at the Jackdaw gas field, Stephen Gethins, the energy minister who proposed the conference motion, issued a statement that failed to say whether the devolved government supported the approval or not.
Mr Swinney made the SNP’s position clear at First Minister‘s Questions earlier this month when he spoke out against a potential ban on new oil and gas developments.
He said he opposed a UK government bid to ban new oil and gas exploration in the North Sea saying each application should instead be assessed on a case-by-case basis as to whether it is better for the climate than importing fossil fuels.
His intervention confirmed the SNP had U turned on its outright opposition to new licences after a gradual shift in the party’s position for several months as it came under pressure to change policy from senior figures in the North East of Scotland.
In his speech after winning Aberdeen Deeside and North Kincardine at the Holyrood election in May, Stephen Flynn underlined his support for North Sea industry workers.
“To those people in our region who work in the oil and gas sector, know that in Scotland’s parliament you will have no greater champion,” he said.
Mr Flynn stood down from his Westminster seat of Aberdeen South after winning the Holyrood seat with a by-election held for the Westminster seat in June.
But the seat fell to the Scottish Conservatives – a first Westminster by-election victory for the party in more than 50 years – with Mr Flynn taking to social media saying it was a result “that some will need to reflect on, quite heavily”.
The newly-elected Tory MP Douglas Lumsden, a former oil and gas worker, said his constituents had sent a message that “the destruction of the oil and gas industry must stop now”.
Mr Lumsden defeated SNP candidate Richard Thomson, a former MP for Gordon, by a margin of more than 6,000 votes, with the Tories taking almost half of all ballots cast.
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But while the shift may be popular among the SNP in the North East of Scotland politicians have been less enthusiastic particularly in parts of Glasgow where the Greens took Ms Sturgeon’s former seat of Glasgow Southside at the May Holyrood election.
“For the avoidance of doubt: like my Southside SNP colleagues, there’s no world in which I would ever support further oil and gas exploration or extraction,” Paul Leinster, the SNP councillor for the city’s Langside ward, which lies in Glasgow Southside wrote on X.
“I support the fastest, fairest transition to renewables and that does not require Jackdaw or Rosebank.”
The Greens are now planning to exploit the SNP disquiet as they step up their campaigns in Scotland’s largest two cities ahead of the local government elections in May.
“We’re going to use John’s answer [at FMQs] for another big fundraising push with members and supporters, setting our sights in picking off SNP seats next year,” said the insider.
A spokesperson for the Scottish Greens said: “We’re delighted to welcome so many new members and to see people across Scotland backing us.
“More and more people are finding a political home in the Scottish Greens because we are making a progressive case for Scotland’s future that works for people and planet.
“This is one of the reasons why we now have eclipsed Scottish Labour, reaching 11,000 members, because every one of them believe in a future for Scotland that has climate justice at its heart.
“While other parties roll back their climate promises and cheer on new fossil fuel projects like Rosebank, we have stood firm on climate action and on shifting the tax burden from work to wealth as people struggle to pay their bills while the richest in our society can afford to contribute more.”
An SNP source said: “While the Greens are focused on taking council seats off the SNP, we are focused on winning independence for Scotland as demonstrated by our £100,000 independence campaign, Choose Scotland.
“And our message is simple.
“Scotland’s energy should be in Scotland’s hands and every single penny from our North Sea resources should be put straight into renewables. A position overwhelmingly supported by the public.
“That is exactly how our Nordic friends operate and it is how we deliver net zero and an independent Scotland.
“The question is why any party that claims to back Scottish independence would sit back and allow our resources to be syphoned off to the Treasury, actively ruin Scotland’s renewable industry and increase our reliance on carbon heavy imports from Donald Trump or the Middle East.
“The SNP is the party of independence and we will never stand by and allow Westminster to rob our resources and give nothing back in return. That is a message we are proud to take to every corner of Scotland, every single day of the week.”