The MP for Skye, Ross, and Lochaber announced his decision to stand down just weeks after appearing to see off a leadership challenge. The decision came a week after Mr Blackford told The Scotsman he planned on being the last Westminster leader of the SNP before Scottish independence.
Stephen Flynn, the Aberdeen South MP and energy spokesperson, is understood to be in pole position to win the leadership contest. He has the backing of a majority of SNP MPs in the race for the top job and is highly likely to comfortably win the vote at the party’s annual general meeting on Tuesday.
This is despite his denial of reports two weeks ago that he had told SNP officials he intended to challenge the long-standing leader of the Westminster group at the annual internal elections, claiming he had “no intention of standing”.
It is understood Mr Blackford spoke to Ms Sturgeon before resigning – a decision accepted by the First Minister following a lengthy conversation about the move. The two SNP figures have a genuine friendship and enjoy a close personal and political relationship.
Insiders said the final decision to resign was made by the MP and suggested Mr Blackford had mentally “crossed the rubicon" following the aborted coup attempt.
No one issue appears to have triggered the move, but the annual election of a Westminster leader is set for Tuesday. It has become apparent Mr Flynn has the numbers and the confidence of a majority of MPs.
Frustrations within the party around a range of issues, including the handling of the sexual harassment allegations made against Patrick Grady, the former SNP chief whip, alongside feelings of detachment from the SNP’s centre of power in Holyrood, drove some towards backing Mr Flynn.
Discontent around the handling of a vote on Tuesday where SNP MPs were given a free vote with a preference on abstention around a move by Conservative MPs to have culture spokesperson John Nicolson sent before the privileges committee was also attributed as a factor.
Party insiders said it is highly unlikely there would be more than one name on the ballot for the election next week. Allies of Mr Flynn said the 34-year-old MP is “considerably hungrier and younger" and would offer a change of pace for the party from Mr Blackford's approach, which has sometimes been accused of being too cozy with the Westminster establishment.
In a statement released abruptly on Thursday morning before First Minister's Questions, Mr Blackford said “now is the right time for fresh leadership” as the party headed towards the "next steps in winning Scotland's independence”.
He is set to take a business engagement role within the SNP's independence campaigning following his resignation, the statement said, alongside his role as MP for Ross, Skye and Lochaber.
Mr Blackford added: "During my time as leader, the SNP won a landslide victory in the 2019 general election, with an increased share of the vote and MPs, and support for independence has continued to grow with polling this week showing a majority in favour.
"I would like to thank our MPs and staff for all their support over the past five years. Whoever replaces me as Westminster leader will have my full support as, together, we stand up for Scotland's interests and democratic right to choose our future in an independence referendum."
In a statement, Nicola Sturgeon paid tribute to Mr Blackford's five year tenure, praising him for the “huge electoral success” and for an “outstanding job in holding the Tory government to account”.
She said: “I would like to place on record my thanks for Ian's diligence, tenacity, friendship and loyalty in his time as group leader.
“I look forward to working with Ian’s successor as group leader in Westminster as we continue to make the case for the people of Scotland to have a democratic choice on the country's future.”
Quizzed later on Mr Blackford’s departure, the First Minister told STV: “No, it’s not a coup. Ian has been Westminster group leader for five years now and he represents one of the furthest flung constituencies in the country.
“He’s making the decision given all that lies ahead for the SNP, given all the exciting work that lies ahead for the SNP, that this is the right decision for him to pass on the baton.”
Joanna Cherry MP, one of Ms Sturgeon’s most vociferous internal critics who was sacked from the frontbench by Mr Blackford, said she was "pleased to hear this”. On Twitter, she wrote: “It's time for fresh leadership and tolerance of debate and diverse viewpoints. I hope the SNP Westminster group will now be left to choose our new leader without outside interference and in accordance with our standing orders.”
Ian Murray, the shadow Scotland secretary, said Mr Blackford's resignation demonstrated the SNP was “in total disarray”. He said the “nats are deserting the sinking ship", adding “Nationalist MPs know Nicola Sturgeon's plan for a de-facto referendum is finished before its even started and are worried about Labour gaining seats. Across Scotland, Labour is growing in strength every single day.”
Christine Jardine, the Scottish Liberal Democrat spokesperson on Scottish affairs, said Mr Blackford “should have stepped aside a long time ago” and claimed her party was “breathing down his neck” in his constituency.
She said: “From his bitter campaign against Charles Kennedy to his mishandling of sexual harassment allegations against an SNP MP, it's clear that Ian Blackford has never been fit to lead.
“It's been clear that his SNP colleagues have been pushing him to go and the recent failed leadership challenge was just one indication of their dissatisfaction.”
Craig Hoy, the Scottish Conservative party chairman, said Mr Blackford had “jumped before he was pushed” and that SNP MPs were not willing to forgive the MP for his handling of sexual harassment allegations.
"His resignation is a total humiliation for Nicola Sturgeon,” he said. “She shamefully stood by her Westminster leader earlier this year despite his appalling handling of the complaints made against Patrick Grady. This showed a total lack of judgement by Nicola Sturgeon.
"Ian Blackford should have been sacked immediately for his disgraceful behaviour, but he was allowed to stay in post for several months and now gets to dictate the terms on which he leaves.
“The SNP Westminster group are clearly in a state of disarray and Nicola Sturgeon is rapidly losing her grip over her party.”