John Swinney has said he wants to work with Sinn Fein to bolster his campaign to break up the United Kingdom.
The First Minister said he would ‘enjoy’ cooperating with the party, which has historic links to IRA terrorism, and with Welsh Nationalists Plaid Cymru to secure Scottish independence.
Their collaboration would ‘change the dynamics’ of the UK ‘irreversibly’, he said.
The Scottish Conservatives said it proved ‘the Union is on the line at this election’.
Mr Swinney has repeatedly said the SNP will win a Holyrood majority in May and it would give him a mandate to secure a new vote on separation.
Plaid, who want independence for Wales, are close to an historic win in the Senedd, with a poll this week putting them level with Reform UK as Labour’s support collapses.
Sinn Fein’s Michelle O’Neill is already First Minister of Northern Ireland and is not due to face an election until next year.
The largest party at Stormont, Sinn Fein wants a referendum on a united Ireland by 2030.
John Swinney says he would 'enjoy' working with Sinn Fein and Plaid Cymru to secure Scottish independence
Scottish Tory deputy leader Rachael Hamilton said: ‘Pro-UK voters will be appalled and terrified at the prospect of John Swinney working in cahoots with Sinn Fein to break up the United Kingdom.
‘This shows his and the SNP’s true colours - they will gladly team up with any party, in any corner of the UK, so long as they share his obsession with tearing it apart.
‘That’s why the prospect of an SNP majority, which Swinney reckons is in the bag, is so frightening.
‘Keir Starmer is so weak he would probably give into his demands.
‘The Union is on the line at this election, and only the Scottish Conservatives can be trusted to defend it.’
Although allied Nationalist governments in Belfast, Edinburgh and Cardiff have been possible since devolution, this year is the first when it has been a realistic scenario.
Ex-Tory cabinet minister Penny Mordaunt has warned the Union could be at risk if Sinn Fein, the SNP and Plaid hold power simultaneously.
She told Sky News in January: ‘There are some real dangers.
‘You could, over the next couple of years, potentially, have three parties in the driving seat in the devolved administrations who are all pro-breaking up the United Kingdom. So us Unionists, we cannot be complacent in this at all.’
At an election event in Edinburgh, Mr Swinney was asked about an SNP majority government working with Sinn Fein and Plaid first ministers to get independence for all three home nations.
He said: ‘I certainly would enjoy the cooperation with counterparts in Plaid and Sinn Fein in Wales and Northern Ireland.
‘I don’t speak for them.
‘But certainly, from my part, I would appreciate that collaboration to change the dynamics of the United Kingdom so that we can… I can secure my objective of winning Scottish independence.
‘I think the United Kingdom would be changed irreversibly if that outcome was to be the case.
‘Michelle O’Neill does not face an election in Northern Ireland at this stage, but she will be in office. And obviously the opportunities for Plaid Cymru are very visible in Wales.’
Asked why three Nationalist governments would change the UK ‘irreversibly’, he said: ‘Because of the nature of the discussions that would then be taking place around the United Kingdom and between the devolved governments and the United Kingdom Government.
‘[It] would change very dramatically with the fact that all those three countries would be led, potentially, by governments committed to fundamental change in the United Kingdom.’
A Plaid Cymru spokesman said: ‘Electing a Plaid Cymru government in Wales and an SNP government in Scotland would result in a powerful bloc - one that would serve as a wake-up call for the UK Labour Government that they would ignore at their peril.’
Pollster Mark Diffley, founder of the Diffley Partnership research agency, said Mr Swinney was glossing over complexities, such as Sinn Fein sharing power with the Unionist DUP and Plaid Cymru being unlikely to win a majority.
But he said a trio of separatist leaders could be a big psychological moment for England and Westminster.
He said: ‘It would be a blow for the Prime Minister.
First Minister of Northern Ireland Michelle O’Neill is not due to face an election until next year
‘Having three Nationalist governments trying to wrangle concessions all the time from a weak Government that’s trying to focus on the economy or war is definitely problematic.
‘Whether it furthers the course of independence in all three countries is another matter.’
Reform UK Scotland candidate Thomas Kerr said: ‘The people of Scotland will be absolutely appalled to hear John Swinney cosy up to Sinn Fein.
‘They harbour ties to some of the most prolific murders and domestic terrorists our country has seen - Swinney is being clumsy at best, and stoking hatred at worst.
‘He should offer an apology and think before he speaks in future.’
Scottish Liberal Democrat campaign chair Wendy Chamberlain said: ‘Aside from John Swinney, the person most excited by the prospect of the UK being destabilised is Vladimir Putin.
Scottish Labour deputy leader Dame Jackie Baillie said: ‘If Swinney spent less time pushing his narrative of grievance and always seeking to blame someone else for his failures, then perhaps Scotland would be in a better place.’
Sinn Fein were contacted for comment.