Scottish Labour leader Kezia Dugdale has pledged to work tirelessly in support of the union if there were to be a second independence referendum.
She made her promise during a speech to her party conference.
Ms Dugdale said that the country still bore the scars of the September 2014 vote.
Earlier, the Perth gathering heard UK Labour's deputy leader Tom Watson talk of his "huge disappointment" the party did not win the Copeland by-election.
By BBC Scotland's political editor Brian Taylor
Ms Dugdale resolutely stressed that she was not keen to engage in a further constitutional battle in Scotland, indyref2.
It was a bad thing. It was other than good. It was not desired by the people of Scotland. It was a mistake. It was not needed. It would not put bread on the table. It was a hissing and a byword.
Having thus denigrated constitutional politics, she proceeded to deliver a speech which dealt, very substantially, with….constitutional politics.
A conundrum? Yes, but perhaps also a dose of realism. Thus far, Labour has spent the time since the 2014 referendum and indeed the 2016 version urging a return to the politics of equality, the politics of economic reform.
Ms Dugdale has reaffirmed her party's opposition to a second referendum.
She added that she was ready to "step up and make the case for the union" once again, if the SNP broke its vow to not hold another contest.
The MSP argued that Labour's "historic values can endure even under this greatest threat" and that it will "always put the people of this country first".
Ms Dugdale added: "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."
Federal power plan
Ms Dugdale also used the speech to announce her party's campaign to create a new Scottish Child Benefit.
The plan could see the monthly sum go up to £20 per child per month by 2020.
She believed the move would help reverse the fall in the value of child benefit and send a strong message that Scotland would not allow hard working families to "bear the brunt of Brexit".
Ms Dugdale highlighted a conference vote in favour of a People's Constitutional Convention to look at establishing a federal power plan for the UK.
She said in the "coming months" she, former Labour prime minister Gordon Brown, and party representatives from Wales and the English regions would begin to set out "how we will take this agenda forward together".
Ahead of Ms Dugdale's speech, Mr Watson told members gathered: "All of us with leadership roles in the Labour Party need to have a long hard look at ourselves at what's not working. Seven years into a Tory government we should not be facing questions about whether we can retain seats we already hold.
"Our job at the next general election is to gain over 100 seats - keeping what we have is supposed to be the easy bit and here in Scotland you have seen what happened when Labour's long standing supporters stop voting Labour.
"We cannot afford to have that happening in England too."
London Mayor Sadiq Khan also spoke on the second day of the three-day conference.
He was forced to clarify his views after using social media to indicate what he would be saying in his address.
'Those who try to divide us'
Mr Khan tweeted an extract which said: "Now's not the time to play on people's fears.
"Or to pit one part of our country - or one section of our society - against each other.
"In that respect there's no difference between those who try to divide us on the basis of whether we're English or Scottish, and those who try to divide us on the basis of our background, race or religion."
That prompted reaction on social media including a response from First Minister Nicola Sturgeon who described Mr Khan's intervention as "spectacularly ill-judged".
She added: "It is an insult to all those Scots who support independence for reasons of inclusion & social justice - the antithesis of what he says."
When he made his speech to the conference he said: "Now of course I'm not saying that nationalists are somehow racist or bigoted - but now, more than ever - what we don't need is more division and separation."