The MP Kenny MacAskill has announced he is quitting the SNP to stand for Alex Salmond's new party at the Holyrood election.
The former Scottish justice secretary said the Alba Party, that launched on Friday, would help achieve independence.
The SNP have called for a by-election in response, and described his departure as "somewhat of a relief".
Mr MacAskill is the most high-profile SNP name to defect to the new party.
He is now the Alba Party's first MP at Westminster and said he would be forming an Alba Party group there. He said the membership of Alba was "going through the roof".
Mr MacAskill told the BBC News Channel: "The gamble isn't going for an independent supermajority, the gamble is remaining in a United Kingdom led by Boris Johnson enforcing austerity and making the world a riskier place.
"This is a chance to deliver the right for Scotland to decide its own future and to reject the right of Boris Johnson to veto Scotland's right to choose."
He said he would continue to serve in East Lothian.
"This is about continuing to drive forward the cause of independence," he said. "I didn't leave the SNP - that I've been a member of for more than 40 years - lightly, but I've been committed to the cause of independence throughout all of my life and it's never been more vital or necessary than now.
"Now is the time for Scotland to choose a different way, a different direction, but to be able to do that we require the powers of an independent nation and that's why Alba was formed and that's what a supermajority for independence will deliver in May."
He added: "This isn't just the Alex Salmond party or even the Alex Salmond and Kenny MacAskill Party, it's a party for those who support the cause of independence and who want to make sure we grasp that nettle which is badly required because of the risks and challenges that we face."
'An increasing embarrassment'
A total of 48 SNP MPs were elected in December 2019, making it the third largest party at Westminster.
News of Mr MacAskill's defection will not come as a surprise to many as the East Lothian MP has long been a close ally of Mr Salmond and has been critical of the SNP's leadership.
The SNP's Westminster leader Ian Blackford said: "He has been an increasing embarrassment to many in the SNP and his departure is somewhat of a relief.
"That he is joining a party with serious questions to answer about its leader's suitability for public office is no surprise.
"He should now resign his seat in the House of Commons to let a by-election take place immediately so the people of East Lothian can elect a new MP who will focus on their interests, rather than self-interest."
Scottish Labour MP Ian Murray said: "The Scottish and Westminster parliaments are not places for people to play at politics and Kenny McAskill must now stand down to allow the people of East Lothian to make his obsession with division history. The people of East Lothian deserve better than to be cast aside without a second thought."
Lib Dem MP Alistair Carmichael added: "Kenny MacAskill wants to go from being the SNP MP for East Lothian, to the Alba party MSP for Lothians. I will be interested to see how he pursues this role from his home in Banffshire."
A spokesman for the Scottish Conservatives said: "Instead of focusing 100% on recovery and rebuilding Scotland, Alex Salmond and Kenny MacAskill's new party are seeking to game the electoral system and boost nationalist seat numbers so they can get an independence super-majority."
A spokesman for the Scottish Greens said: "This election should be about putting forward a positive vision for Scotland's future, not settling personal vendettas. Kenny MacAskill has simply joined the public meltdown of a man disgraced by his own behaviour."