Nicola Sturgeon has claimed Kezia Dugdale promised that Labour would not stand in the way of a second referendum during a private conversation last year.
The SNP leader claimed the Scottish Labour leader said she would drop her opposition to another poll during a stormy leaders’ debate hosted by STV.
According to Ms Sturgeon, Ms Dugdale made the offer in the aftermath of the Brexit vote last June.
The SNP leader said a referendum would give Scotland a “choice” between independence and a hard Brexit.
She added: “You [Dugdale] used to agree with me on that. You and I spoke the day after the EU referendum and you told me then that you thought the change with Brexit meant that Labour should stop opposing a referendum.”
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The claim made less than 36 hours before voters go to the polls for the General Election on Thursday, was disputed by Labour.
And Ms Dugdale went further tweeting after the debate that it was a “categoric lie”.
Nevertheless, the claim risks severely undermining the credibility of Labour’s strategy on the constitution.
Ms Dugdale has made great play of her opposition to another vote in the run-up to the General Election, despite Jeremy Corbyn suggesting he would be open to talking with the SNP on the issue should he make it to Downing Street.
Ms Sturgeon’s account of the discussion was described as a “bombshell” by the Scottish Tories who have based their campaign on being the party most implacably opposed to independence.
Ms Sturgeon’s claim was pounced on by Scottish Conservative Ruth Davidson during the hour and half “Scotland Debates” programme broadcast in front of a 130-strong studio audience in the Tron Theatre, Glasgow.
Ms Davidson asked Ms Sturgeon: “Did you just tell people you had a private conversation with Kez Dugdale last June where she said she was going to drop Labour’s opposition to independence?”
Ms Sturgeon replied: “She said that she thought that Brexit changed everything and that she didn’t think Labour could any longer go on opposing a second independence referendum.
“She was entitled to change her mind, I accept that but what I don’t think any politician is entitled to do is to deny people in Scotland a choice about their own future.”
Last night the Scottish Conservatives attempted to capitalise on the revelation as campaigning reaches its final stages.#
The party’s constitution spokesman Adam Tomkins said: “This is a bombshell revelation which holes Scottish Labour’s entire campaign below the waterline.
“Kezia Dugdale has spent this campaign claiming she opposes a second independence referendum, now we learn she’s been having private chats with Nicola Sturgeon about her support for it.
“It is an utter disgrace and it proves that the only pro-UK vote at this election is for the Scottish Conservatives.”
Ms Sturgeon’s account of her private conversation with Ms Dugdale was disputed by the Labour leader, who described it as “absolute nonsense”.
Ms Dugdale admitted that a discussion had taken place about Brexit.
But she said: “The idea that I would do anything other than protect the United Kingdom is absolute nonsense.”
Later a Labour spokesman said: “This is a lie from Nicola Sturgeon. It is insulting and demeaning to the office of First Minister. It is nothing but a final act of desperation from an SNP leader who knows the public has turned against her. It shows how far she is prepared to go in the hope of electing a Tory government.
“Kezia Dugdale campaigned tirelessly against independence in the 2014 referendum. She has always campaigned tirelessly against a divisive second independence referendum. The Labour Party will never support independence because of the turbo-charged austerity it would inflict on working people in Scotland.”
The final Scottish debate of the campaign was held last night following its postponment in the aftermath of the terrorist attacks on Manchester.
In the opening exchanges were dominated by the question of a second independence referendum with Ms Sturgeon facing calls to cancel her plans for a second vote.
The pro-Union leaders all promised that a vote for their parties would take indyref2 off the table and warned of consequences of holding another one.
Ms Sturgeon said the SNP would stand up for Scotland and campaign against an extreme Brexit.
She argued that voting Labour or Lib Dem risks splitting the anti-Tory vote.
Ms Davidson said every single MP would stand up for Scotland. She said the SNP was more interested in stoking up more division than concentrating on education.
Ms Dugdale said the election was chance to get rid of the “misery” of the Tory Government while opposing a second referendum.
“We have a chance on Thursday to get rid of a mean, miserly, incompetent Tory Government, and replace it with a Labour Government,” Ms Dugdale said.
“A Labour Government that will deliver a £10 minimum wage, £3billion more for Scotland’s budget, dignity for the disabled, and a National Investment Bank to invest in our economy.
“All that is on the ballot paper alongside opposition to a second independence referendum if you vote Labour on Thursday.”
Mr Rennie said Ms Sturgeon should take a second referendum “off the table” arguing that the oil crisis had created a £15 billion spending gap and education standards had fallen down international league tables.
“Are you going to cancel another independence referendum,” Mr Rennie asked.
Ms Sturgeon countered by saying it was “ridiculous” that the Lib Dems were against another independence referendum, but wanted another Brexit vote.
The SNP leader claimed it was the other parties who were “obsessed” with a second vote.
Ms Davidson pointed out that a year ago Ms Sturgeon had told a similar STV debate and said there would not be another vote unless public support for independence increased.
Ms Sturgeon said that since then the Brexit vote had changed everything. The SNP leader said Ms Davidson had warned how damaging Brexit would be when she had campaigned to stay in the EU.
Now, Ms Sturgeon said, Ms Davidson had been told by Theresa May that “Brexit was the greatest thing since sliced bread and she has fallen into line.”
Later Ms Dugdale and Mr Rennie ganged up on Ms Davidson to denounce her as a champion for a hard Brexit”.
Questioned by Ms Davidson, Ms Davidson defended Jeremy Corbyn claiming her would be effecitve at standing up for the Union.
Ms Sturgeon attacked Mr Corbyn accusing him of failing to stop welfare cuts that “would drive children into poverty and that is indefensible”.
Ms Davidson came under attack over the UK Government’s two child cap and its associated “rape clause”.
Ms Sturgeon, Ms Dugdale and Mr Rennie combined to criticise Tory policy that requires rape victims to fill in a form to claim benefits for a third child conceived as a result of a sex attack.
Ms Sturgeon asked how Tories could “sleep at night” and said the £380 million used to finance a tax cut for the rich should be used to scrap the rape clause.
Ms Dugdale said the policy showed the Tories were “just as callous and heartless as they have always been. The rape clause is unfit for a civilised society.”