KEZIA Dugdale has ruled out another cross-party Unionist campaign along the lines of Better Together if there is a second independence referendum.

After thousands of Labour supporters deserted the party when it allied with the Tories in 2014, the Scottish Labour leader said she “cannot imagine” it ever happening again.

At the Scottish Labour conference in Perth, her deputy Alex Rowley also said Labour would “never stand on any platform with the Tory party” because of its austerity policies.

Nicola Sturgeon has said a second referendum is "all but inevitable" if Scotland is dragged out the EU single market by Brexit.

Ms Dugdale will today attempt to sharpen up her party’s position on the constitution by saying Scottish Labour will "never" support independence while she is leader.

She has previously ruled out support only for the current parliament, which runs to 2021.

Squeezed between the SNP and Tories, Scottish Labour yesterday adopted a third way on the constitution: a federal plan to give Holyrood vastly more powers within the Union.

Delegates backed Ms Dugdale's idea of a Labour-led "People's Constitutional Convention" to devise a new federal UK and a “new Act of Union” by 2020.

Speaking at a fringe event later, Ms Dugdale said if there was another referendum, there was a case for putting a third option on the ballot paper for federalism.

She said: “I think the federal solution is the right one for Scotland, in terms of respecting the result of the 2014 referendum - a strong Scottish parliament in the United Kingdom.

“I don’t want there to be another referendum, but I can certainly see a legitimate case that if there were to be a referendum, that you might put something as strong as a federalist solution within the United Kingdom on the ballot paper.”

Asked if there would be another Better Together, she said: “I cannot imagine that happening.”

She added it would be a “huge mistake” if Scottish Tory leader Ruth Davidson led a future Unionist campaign.

Instead of a cross-party campaign, there could be a non-political one along the lines of Stronger In in last year’s EU referendum.

She said: “ I would be arguing very strongly for a Labour case for the Union should we find ourselves in that position.

“In a referendum campaign, for legal and financial reasons, there can only be two sides, and you would find different politicians from different parties in those two sides. But that’s very different from having a joined-up campaign with another political party.”

Ms Davidson said: "On the day Labour is licking its wounds over its humiliation in the Copeland by-election, the Scottish party decides the way to reconnect with the real lives of ordinary families is a sleep-inducing plan for People's Constitutional Convention and yet more upheaval to our precious union. This is a party that doesn't know when to stop digging.”

Former LibDem Scottish Secretary Alistair Carmichael congratulated Labour on their “long, slow walk to federalism”, adding: “We need a thorough refresh of the UK system of governance. There needs to be a Constitutional Convention – once again well-established Lib Dem policy. It’s a shame Labour have taken so long to get to this point.”

An SNP spokesman added: “SNP spokesperson: "Labour would prefer decades of Tory rule in a hard Brexit Britain than allowing Scotland to choose our own future. That's exactly the attitude that has caused voters to abandon Labour in their droves."

Deputy UK Labour leader Tom Watson will today back the federalism plan and say all the nations and regions of the UK should get more powers.

He will say: “I want us to join together to rewrite the British constitution for the new era in which we find ourselves. In this post Brexit world I favour devolution all round. I favour the regions and nations taking from Brussels: powers over Agriculture and Fisheries

regional policy, social Funds, environmental protection. And sharing control of the £4bn worth of spending by Europe in the UK. We can unite together on a radical programme of constitutional change for the regions and nations of the UK.”