The SNP has said the departure of Kenny MacAskill is "somewhat of a relief" after he quit to join the newly-formed Alba Party.
Alex Salmond launched his latest political project on Friday as a means to create a "supermajority" of pro-independence supporters in Holyrood.
Former Scottish justice secretary Mr MacAskill is the first big SNP name to defect to the new party, although former SNP councillor Chris McEleny is standing as a candidate for the Alba Party in the upcoming Scottish Parliament elections.
The National reports that the East Lothian MP wrote to his staff: "I will be joining the newly-formed Alba Party to deliver a supermajority for independence through the list vote and which I believe is essential to achieving our nation's independence."
The SNP have called for a by-election in response, and described Mr MacAskill's departure as "somewhat of a relief".
The party's Westminster leader Ian Blackford said: "After yesterday's events this is the second least surprising news in Scottish politics.
"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."
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."
Meanwhile, it appears former SNP MP Corri Wilson has also defected to the Alba Party.
Her updated Twitter profile describes herself as an "Ex SNP Cllr & MP for Ayr Carrick & Cumnock" while she also retweeted a post that quoted her as saying she would stand for the newly-formed party in South of Scotland.
It said: "I am delighted to be joining Alba Party whose goal is to secure independence for Scotland, the cause that so many in Scotland have given so much to support."
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