Scottish Labour leader Kezia Dugdale will pledge to work tirelessly in support of the union if there were to be a second independence referendum.
She will make the promise during a speech later to her party conference.
Ms Dugdale is expected to say that the country still bears the scars of the September 2014 vote which resulted in a “no” to independence.
The second day of the Perth conference will also hear from UK Labour’s deputy leader Tom Watson.
He is due to argue that a number of powers, such as agriculture, fisheries and environment protection, should be devolved to Scotland when the UK leaves the European Union.
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Ms Dugdale, who will address her conference in the afternoon, will reaffirm the party’s opposition to a second referendum.
But she will add that she is ready to “step up and make the case for the union” once again if the SNP breaks its vow to not hold another contest.
The MSP will also argue that Labour’s “historic values can endure even under this greatest threat” and that it will “always put the people of this country first”.
‘Time Sturgeon changed her tune’
Ms Dugdale’s speech comes a day after the party adopted a policy in favour of a federal UK.
She will add: “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.
“It’s time Nicola Sturgeon changed her tune.
“I want the first minister of Scotland to focus on the Scottish NHS, on our economy and on our schools. I don’t want a first minister whose priority is the constitution.”
BBC political reporter Philip Sim from the conference
At one point on day one of the Scottish Labour conference, a steward asked me if I was lost – adding that “a lot of people are looking like that way since last night”.
It suggests a hangover from the by-election result in Copeland, or perhaps a particularly heavy pre-conference party.
The theme of the conference, in case you hadn’t spotted it plastered across more or less every available surface, is “Together We’re Stronger”.
It’s a slogan which underlines the extent to which the event has found itself, once again, dominated by constitutional issues. The constitution of the UK – and that of the Labour Party itself.
Kezia Dugdale has put federalism top of her own agenda. Tom Watson will continue that theme today, as the UK and Scottish parties strive to look Stronger Together. London Mayor Sadiq Khan has also been jetted in as part of the same effort.
And beyond the internal politics, the issue of independence also continues to hang over Labour.
Ms Dugdale will use her speech today to reaffirm her party’s backing for the United Kingdom, which she reckons is also Stronger Together.
Many members, including Ms Dugdale herself, are less than happy with the ongoing constitutional focus; they would far rather discuss more domestic policies.
But as Cat Headley pointed out on the stage, as much as the party desperately wants to move on from the binary issues, “the country isn’t ready to”.
Scottish Labour’s Dugdale pledges to work ‘tirelessly’ for union}