Nicola Sturgeon will threaten to derail Brexit by setting out plans for a second independence referendum unless Theresa May offers Scotland a special deal.
Ms Sturgeon has previously hinted that autumn 2018 would be a suitable time to call a referendum.
The ultimatum is expected to be delivered on Monday morning with the intention of influencing a Commons vote on Monday on MPs being given a meaningful say on the final deal offered to Britain by the EU.
She also wants to pile pressure on Mrs May just 24 hours before t he Prime Minister hopes to be in a position to formally trigger Article 50.
If the Article 50 Bill is passed, it could receive royal assent as early as Tuesday, clearing the way for Mrs May to begin the two-year Brexit process by informing the EU of Britain’s decision to leave.
But Ms Sturgeon wants the Prime Minister to include in her letter to the EU a series of demands for Scotland to be given special treatment in the Brexit negotiations.
Downing Street has refused to bow to any such demands and is highly unlikely to alter its plans unless facing a parliamentary rebellion over the issue. It comes as:
• Boris Johnson insisted Britain would be “perfectly OK” without a deal on Brexit and should be prepared to walk away from EU talks without one;
• A Tory rebellion on Article 50 that would have seen a Lords amendment enshrined in law appeared to be faltering;
• MPs affected by rail strikes on Southern, Merseyrail and Arriva Trains North were told to drive to Parliament so they do not miss the crucial vote;
• Theresa May prepared to use Commonwealth Day to stress the importance of the 52-nation organisation to the UK’s post-Brexit economy.
Ms Sturgeon has called a press conference for Monday morning at Bute House, her official residence in Edinburgh, but in a highly unusual move decided not to make an official announcement of it.
Downing Street on Sunday appeared unaware that the event was taking place, suggesting the SNP intended to spring a surprise for maximum impact.
The First Minister is also likely to be hoping it will bolster support for the Article 50 amendment in Monday night’s vote, by suggesting to MPs that they could help avoid the break-up of the Union by using a “meaningful” vote on the Brexit deal to give ground to the SNP.
Ms Sturgeon wants to keep Scotland in the EU single market even if the rest of the UK leaves, which would involve the devolution of nearly every policy area except defence and the macro-economy.
With the UK Government highly likely to reject her proposals, it is thought the First Minister will attempt to blame Mrs May for pushing her towards a second referendum.
The First Minister said last week that autumn 2018 is her preferred time for another vote, when the shape of the Brexit deal becomes known but before the UK leaves the EU.
However, senior Tory sources have said Mrs May is unlikely to allow a vote to be staged until after Brexit takes place in March 2019. They are also wary of holding a vote in the immediate aftermath as this would still allow an independence campaign to be conducted in parallel with the Brexit talks.
Ms Sturgeon has been given a boost by opinion polls which show Scotland divided 50/50 on the question of independence, compared with the 2014 referendum result which favoured staying in the Union by 55 per cent to 45.
Jeremy Corbyn, the Labour Party leader, made his own intervention in the debate by saying it would be absolutely fine for Scotland to hold a second poll. Ian Murray, Labour’s only MP north of the border, reacted by angrily accusing him of destroying the party.
David Davis, the Brexit Secretary, tried to reassure Tory MPs considering defying the whip on the vote that it is inconceivable they will not get a say on the outcome of the negotiations.
In a final plea to the rebels, he said the BBC’s Andrew Marr show: What I don’t want to do is take a simple Bill which is designed to do nothing more than put the result of the referendum into law… please don’t tie the Prime Minister’s hands in the process of doing that for things which we expect to attain anyway.