The Labour Party will "never betray its values" by backing Scottish independence, the party's leader north of the border is set to insist.
Kezia Dugdale will pledge that while she remains in the job, the party will "never support independence" and she will "work tirelessly" to keep Scotland in the UK if there is a second referendum on the Union.
She will make the commitment in her speech to the Scottish Labour Party conference in Perth, which will also hear from London Mayor Sadiq Khan and UK deputy leader Tom Watson.
The event is taking place as UK leader Jeremy Corbyn is facing calls to consider his position after defeat in the Copeland by-election, which saw the Tories seize the Cumbrian seat in what was the first gain for a governing party in a parliamentary by-election since 1982.
In Scotland, which was once a Labour heartland, the party has now slipped behind the Conservatives to become the third largest party at Holyrood.
The SNP remains the largest party in the Scottish Parliament, and with six pro-independence Green MSPs, there is a majority there in favour of Scotland leaving the UK.
Ms Dugdale, whose party has now formally endorsed her policy for a federal UK with a "new Act of Union", will today insist: "The Labour Party I lead will never support independence.
"It's not an escape from Tory rule, it's not an escape from Brexit, it's not an alternative, it's the same old song that the SNP has been singing for decades."
Scotland "still bears the scars" of the 2014 independence referendum, in which voters backed staying in the UK by 55% to 45%, Ms Dugdale will add.
While Labour in the Scottish Parliament will "never support" a fresh ballot on this, she will say: "The unfortunate truth is these - with the SNP supported by the Greens they have a majority in the Scottish Parliament.
"So, if this country faces another referendum at any point in the future, I will work tirelessly to make sure that our side of the argument is successful again."
The Scottish Labour leader will use her speech to the conference to restate her opposition to both independence and a second referendum.
"Some people, inside and outside the Labour Party, still say that we should back independence. Not because it is right, but because they think it is popular," she will say.
"Let me say this to them - the Labour Party I lead will never betray its values so easily.
"It will never turn its back on the idea that together we're stronger.
"That we can achieve far more working in partnership than we can alone, that our historic values can endure even under this greatest threat."
Meanwhile, Mr Watson will argue that the party in Scotland is leading the way by "proposing new powers for the Parliament" as part of the new constitutional arrangements.
He will tell the conference: "Scottish Labour says that instead of repatriating powers from Brussels to Westminster, they should be handed to the nations and regions.
"And so, it is Labour, from Scotland that has led the way for the last twenty-five years in reframing the British constitution - and it is doing so again today.
"I want us to join together to rewrite the British constitution for the new era in which we find ourselves."
The party's proposed constitutional convention, to look at how powers should be distributed among the nations and regions, is "not a talking shop about arid constitutional issues", Mr Watson will insist.
Instead, he will say: "It's about power to the people. Economic power, financial power, political power. It's about how we can unleash the economic potential of every part of the UK.
"How we can transfer power from the centre to the regions and nations. How we can ensure a fair allocation of resources based on need.
"And that means power to all of us - here in Scotland, in the North East, the North West, Yorkshire, Wales - power to the north."
The deputy leader will declare that the party in Scotland is once again "shaping the future, not because Scotland is leaving the UK but because Scotland is leading the UK".