Kenny MacAskill has quit the SNP to join Alex Salmond’s Alba party.
The East Lothian MP announced the decision to join the new party on Saturday.
He wrote on Twitter: “Alba party will worry the SNP and Greens, but appeal to many independence voters.”
Alba was officially launched by former first minister Salmond on Friday and will contest the list section in May’s election.
Ian Blackford is now calling for a by election to contest MacAskill’s seat as he called the MP an ‘increasing embarrassment’.
The party’s Westminster leader 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.”
Ian Murray, the shadow secretary of state for Scotland, said: “The already thin veneer that separated Alex Salmond’s party from the SNP has totally cracked in just 24 hours.”
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.”
In an open message to Scottish Labour and the Lib Dems on Twitter Scottish Conservatives leader Douglas Ross said: “The SNP and Salmond’s party are aiming for an independence super majority.
“Let’s meet the new threat of indyref2 with an equally strong response. We are still better together so lets work together.”
Scottish Labour leader Anas Sarwar said on Friday: “The people of Scotland deserve a parliament focused on national recovery, not SNP infighting.”
The Scottish Greens said in a statement: “This election should be about putting forward a positive vision for Scotland’s future, not settling personal vendettas.”