Party leader Richard Leonard pledged to stay in his post following the departure of Neil Findlay, who ran the party’s European election campaign, and justice spokesman Daniel Johnson.

And Mr Leonard said he hoped Scottish Labour would now adopt a policy to support a second referendum on Brexit with the option of remaining in the European Union.

A party spokeswoman confirmed Labour’s Scottish Executive Committee will now vote on a policy change regarding Brexit when it meets on 8 June. She added that a decision to not only support a second Brexit referendum, but also to campaign for Remain in such a vote could be taken regardless of what UK Labour’s policy is. She said: “If there is a second vote then not only will we back Remain but Richard will publicly campaign for it.”

That announcement followed showdown talks between the party group sparked by the exit of Mr Findlay, who also revealed he will leave Holyrood at the next election in 2021.

The resignation is a blow for Mr Leonard, marking the departure of a close ally and one of the most senior backers of Westminster leader Jeremy Corbyn in Scotland.

Mr Findlay hit out at the party infighting and “self-inflicted harm” in his resignation letter to Mr Leonard. Mr Johnson said he was resigning from the frontbench over the party’s position on Europe.

The resignations come a day after the party saw its worst-ever result in the European elections in Scotland, losing its two MEPs and dropping from second to fifth in the polling, provoking party MPs Ian Murray and Martin Whitfield to publish a stinging statement in this newspaper yesterday criticising the European election strategy and the leadership’s approach on Brexit.

Mr Findlay’s resignation letter called for the party to “end the eternal, internal fighting within our party and the toxic culture of leaks and briefings that come from some within the Scottish and UK parliamentary groups”.

He said: “This self-inflicted harm has to stop. Our party leaders and our leadership teams deserve so much better.

“Scottish Labour will win again when the focus is on the public and not on the internal politics of the parliamentary group.”

A Labour source said Mr Findlay’s role in the European elections defeat made his position untenable.

They said: “Neil led the party to its worst-ever election defeat, so it’s right that he chose to resign before he was sacked. But it’s an absolute cheek for him to call for an end to infighting when nobody has done more to cause internal chaos and create a toxic atmosphere than Neil Findlay himself.”

Mr Findlay played a key part in the pro-Corbyn faction of Scottish Labour that took control from colleagues seen as more centrist after Mr Leonard was appointed successor to former leader Kezia Dugdale. She quit amid complaints she was being undermined by left-wingers – but now Mr Findlay has suggested that centrists are employing the same tactics.

The Lothians MSP said the party was “deeply divided” because many have “never accepted” Mr Corbyn as leader UK-wide or Mr Leonard as leader in Scotland.

Mr Findlay said he had been considering resigning for six months and made his final decision in March.

He said: “It has been an enormous privilege to serve the Scottish Labour Party, first as a councillor and then a Member of the Scottish Parliament.

“I am proud of the work I have done representing my constituents in the Lothian region and of the campaigns I have led in Parliament.”

Mr Johnson made it clear in his resignation letter that the leadership’s position on Brexit was behind his decision to stand down.

He also called for a “step change” in approach from Mr Leonard to the party.

Mr Johnson said: “I represent a constituency that voted 80 per cent in favour of Remain, where over 20,000 people signed the revoke Article 50 petition and where the electorate voted overwhelmingly for parties that were clearly remain and pro-Europe.

“My constituents are clear not only that we must have another referendum, but we must make every effort to ensure the UK remains a member of EU.”

Mr Leonard yesterday played down talk of turmoil in the party and insisted he had the support of his parliamentary group after a second meeting last night.

He said: “I hope that this will become the adopted position of the Scottish Labour Party, that we will support a confirmatory referendum on any deal and … give people the option of voting to Remain.”

Scottish Conservative MSP Annie Wells said Scottish Labour was in “total chaos”. She said: “Scottish Labour has once again come out of an election and decided to form a circular firing squad.”

Richard Leonard vows to support second Brexit vote as senior Labour MPs quit