The SNP's Lisa Cameron has announced her defection to the Conservatives.
The East Kilbride, Strathaven and Lesmahagow MP was facing a selection contest to remain as the SNP's candidate at the next general election.
She said she quit because of a "toxic" culture in the party Westminster group.
Prime Minister Rishi Sunak and Scottish Tory leader Douglas Ross both welcomed her to the party. The SNP has called for Dr Cameron to step down to allow a by-election.
While wishing her well on a "personal basis", an SNP spokesperson said her constituents would be "appalled" they are now represented a Tory MP.
Rishi Sunak said he was "delighted" to welcome a "brave and committed" MP to his party.
"Lisa is right that we should aim to do politics better, with more empathy and less division and a dedication to always doing what we think is right," he said.
Dr Cameron, a former NHS clinical psychologist, said she had received support from Mr Sunak after her mental wellbeing deteriorated in recent weeks, but had no contact from the SNP leadership.
She rowed back on her support for Scottish independence, describing it as divisive. She said she would instead focus on "constructive policies".
The MP claimed previously that she had been "ostracised" by the SNP after speaking out over the handling of allegations against former Westminster chief whip Patrick Grady.
At the time, Dr Cameron did not rule out standing down and triggering a by-election if she did not win the SNP nomination.
The MP, who was challenged by party staffer Grant Costello, had been due to find out the results of the selection contest on Thursday.
SNP sources have told the OceanNewsUK she was expected to lose.
SNP Westminster leader Stephen Flynn said her claims of being ostracised "didn't reflect" his experience of the situation in Westminster.
In a statement, Dr Cameron said: "I do not feel able to continue in what I have experienced as a toxic and bullying SNP Westminster group, which resulted in my requiring counselling for a period of 12 months in Parliament and caused significant deterioration in my health and wellbeing as assessed by my GP including the need for antidepressants.
"I will never regret my actions in standing up for a victim of abuse at the hands of an SNP MP last year, but I have no faith remaining in a party whose leadership supported the perpetrator's interests over that of the victims and who have shown little to no interest in acknowledging or addressing the impact."
'Healing divisions'
The MP said she was "particularly grateful" to the prime minister, praising his "positive, inclusive leadership".
She went on to claim families, including her own, have "experienced significant division regarding the issue of independence".
Dr Cameron added: "This has taken its toll and I have come to the conclusion that it is more helpful to focus my energies upon constructive policies that benefit everyone across the four nations of the UK, and to move towards healing these divisions for the collective good."
Douglas Ross welcomed the new MP to his party.
The Scottish Tory leader said: "Like many ex-SNP supporters, she has realised that her former party is hopelessly divided under Humza Yousaf and incapable of focusing on the real priorities of the Scottish people."
Scottish Labour MP Ian Murray said: "This bizarre move shows that the SNP is falling apart before our eyes."
