Ian Blackford has quit as the SNP’s leader at Westminster.
The shock move comes amid in-fighting among the party’s MPs.
He is tipped to be replaced by Stephen Flynn, who mounted an unsuccessful coup attempt against Blackford two weeks ago.
Blackford’s departure is also a huge blow to Nicola Sturgeon’s authority, as he was one of her closest political allies.
Reacting to his resignation, Sturgeon paid tribute to Blackford’s “diligence, tenacity, friendship and loyalty”.
Blackford, the MP for Ross, Skye and Lochaber, became the SNP’s leader at Westminster in 2017.
His resignation comes just days before the SNP Westminster group’s annual general meeting on December 6, at which he was set to face a leadership challenge.
In a statement, Blackford said “right now is the time for fresh leadership”.
“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,” he said.
“While I am stepping down as Westminster leader, I will continue in my role as the MP for Ross, Skye and Lochaber, and I have also accepted a new role at the centre of the SNP’s independence campaign, leading on business engagement.
“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.”
SNP MPs have been unhappy with Blackford’s leadership of the party’s Westminster group for some time.
In a sign of their disquiet, 16 of them rebelled against the SNP whip in a Commons vote two days ago over whether one of their MPs, John Nicolson, should face a probe by the Commons watchdog for sharing correspondence with Speaker Lindsay Hoyle on social media.
They had been told the abstain on the vote, but instead voted against the move.
Blackford’s leadership was dealt another blow last week when it emerged that Stephen Flynn, the SNP MP for Aberdeen South, was preparing a leadership challenge.
It is understood the coup was thwarted by Sturgeon’s office, who were keen to keep Blackford in place.