Alex Salmond has quit the SNP amid allegations of sexual harassment.
Two complaints were raised in January against the former first minister, who fiercely denies the allegations.
Mr Salmond is taking court action against the Scottish Government to challenge the complaints process activated against him, with papers lodged at the Court of Session in Edinburgh on Tuesday.
Earlier tonight, the former Gordon MP posted a video statement online to announce his resignation from the SNP after 45 years.
He also launched a crowdfunder to help meet the costs of his legal challenge, and within two hours it had exceeded £20,000.
Mr Salmond said he had tendered his resignation to avoid “internal division” within the SNP if First Minister Nicola Sturgeon bowed to political pressure to suspend his membership.
He also said he would not allow further attacks on his party by allowing the debate over his status to continue once Parliament reconvenes next week.
But political opponents rounded on Mr Salmond and said he had “jumped before he was pushed”.
Ms Sturgeon has come under increasing pressure to suspend her mentor and friend while police review the complaints against him.
She said on Sunday she had been aware of the allegations for “some time” and that it had been “extremely difficult to come to terms with them” – but that they “could not be ignored or swept under the carpet”.
Mr Salmond said he “carefully” read Ms Sturgeon’s statement, in which she made it clear the SNP had not once had any complaints about his conduct in the past.
He said: “It seems obvious that Nicola feels under pressure from other political parties to suspend me from SNP membership given recent party precedents. For my part, I’ve always thought it a very poor idea to suspend any party member on the basis of complaints or allegations. ‘Innocent until proven guilty’ is central to our concept of justice.
“However, I did not come into politics to facilitate opposition attacks on the SNP, and with Parliament returning next week I have tendered my resignation to remove this line of opposition attack.
“Most of all, I am conscious that if the party felt forced into suspending me, it would cause substantial internal division.”
But Mr Salmond’s Gordon successor, Conservative Colin Clark, claimed his decision to go showed he had lost “all faith in the SNP machine”.
He said: “Alex Salmond has jumped before he was pushed. There is civil war in the SNP, with many in the party believing that this is a witch hunt.
“Mr Salmond deserves a fair hearing and we should all remember he is innocent until proven guilty. However, he appears to have lost all faith in the SNP machine.”
But Mr Salmond said in his statement that it was his “absolute intention” to
reapply for membership once he has cleared his name, and urged supporters
not to relinquish theirs.
He said his primary focus is now the judicial review to challenge the Scottish Government’s handling of the complaint – and, having received messages of support from across the political spectrum, revealed he has set up a crowdfunding page to help him do that.
“My intention is to secure fairness, because that is necessary to clear my name,” he said.
“The costs of a judicial review in the highest court in the land are huge. Many have asked how they can help directly. Therefore I have established a
crowdfunder to assist with costs.”
Any money left over from the case will be donated to good causes, he said.