Scottish Labour will go to its members to ask whether the party should reverse its stance and support the holding of a second independence referendum.
Richard Leonard will put forward proposals for a consultation on indyref2 at a meeting of the party’s executive on Saturday, opening the door to a historic u-turn.
The party’s stance could be decided by members at a special conference as early as April. It is understood that all options will be left on the table, including Labour backing a multi-option referendum that includes a question on a fully federal UK.
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The Scottish Conservatives said the move completed “Labour’s long and painful surrender to the SNP”.
“This move is a disgraceful sell out of the two million No voters, many of whom identified as lifelong Labour supporters,” Tory chief whip Maurice Golden said.
Mr Leonard’s move promises to deepen the row over indyref2 within Scottish Labour, with the party’s only MP Ian Murray - who is running for deputy UK leader - strongly opposed to a change in position.
The debate encroached on the Labour leadership contest yesterday, with two of the MPs vying to replace Jeremy Corbyn clashing over whether a second independence referendum should take place.
Clive Lewis said his party must not block another Scottish referendum and called for his colleagues north of the Border to be free to campaign for a Yes vote if they wished, warning that Scots should not be “dictated to”.
But Mr Lewis’ stance drew a sharp reaction from leadership rival Jess Phillips, who insisted there were “no circumstances where I think it would be better for Scotland to leave the UK”.
With Labour leadership contenders having until Monday to secure the nominations of 22 MPs, Ms Phillips joined Lisa Nandy and shadow business secretary Rebecca Long-Bailey in meeting the threshold to remain in the contest.
Frontrunner and shadow Brexit secretary Sir Keir Starmer had 59 nominations as of yesterday afternoon, ahead of Ms Long-Bailey on 26, and Ms Phillips and Ms Nandy both on 22.
Shadow foreign secretary Emily Thornberry has seven, while Mr Lewis - seen as an outsider in the contest - has just four.
The shadow financial secretary to the Treasury said the debate over who should lead Labour in the wake of its disastrous general election result had paid too little attention to Scotland.
“It is little surprise... that many Scots see themselves not as partners in a union of equal nations, but as a country shackled instead to a dysfunctional political system that is costing them dearly,” Mr Lewis wrote in a column in The National.
“Given the option to exit the UK, it is little wonder that so many now support independence and, given the prospect of at least five years of Tory rule imposing a Brexit that Scotland did not vote for, the question of independence and a second referendum is unavoidable.”
He added: “It is not for me, as an English MP for an English constituency, to dictate to Scotland what that form of government should be, and there should be no question of Labour opposing a second independence referendum if there is a mandate to hold one.”
Responding to Mr Lewis’ comments, Ms Phillips said: “I care as much about kids in Glasgow as I do my own kids in Birmingham.
“We should be talking about things that actually matter to them: the SNP’s education crisis and rising waiting times.
The Birmingham Yardley MP added: “There are no circumstances where I think it would be better for Scotland to leave the UK.”
The SNP welcomed Mr Lewis’ support for Scotland’s right to hold an independence referendum.
“If front bench spokespeople like Clive Lewis can respect Scotland’s right to self-determination, and understand why so many people are moving to support independence, there is no democratic reason why the UK government should attempt to block a referendum,” SNP deputy Westminster leader Kirsty Blackman said.
Meanwhile, former Scottish First Minister Lord Jack McConnell waded into the Labour leadership contest, backing outsider Ms Nandy calling on his party to find a middle way on the constitution.
The peer criticised the “knee jerk nationalism and knee jerk unionism” of the Scottish independence debate, and said Labour should be capitalising on the SNP’s domestic record, comparing the Scottish Parliament to an “incompetent” developing democracy for the “disgrace” of having passed four major pieces of legislation last year.
Lord McConnell, who was leader of Scottish Labour and First Minister from 2001 to 2007, said he had stayed out of party elections and not made an endorsement in the 13 years, but was inspired by Ms Nandy’s performance at a hustings event this week.
“On Tuesday night Lisa Nandy outperformed the other candidates. She was not only thoughtful but she actually conducted herself and presented herself like an inspiring leader and a potential prime minister,” Lord McConnell told the BBC’s Politics Live programme.
“She’s got this concept of a new ‘red bridge’ between the metropolitan areas where Labour’s doing very well at the moment and the towns and coastal areas and some of the rural areas and post industrial areas, that concept is brilliant.”
Lord McConnell added: “She’s the only one the Tories fear.”
The former Scottish Labour leader also hit out at Mr Lewis’ comments on indyref2.
“Clive said on Tuesday night basically that Labour should give up with Scotland and have a pact with the SNP and make them the equivalent of the Scottish Labour Party, that’s not a solution to this situation,” he said.
“We need to look clearly at what we’ve been saying and doing, start afresh, and rebuild the case for social justice.
“There were four bills [passed] in the Scottish Parliament last year… It’s an absolute disgrace. If it was a democracy in a developing country somewhere, we would be saying it was incompetent.”