Dr Lisa Cameron has joined the Scottish Conservatives from the SNP due to a "toxic and bullying" culture within Scotland's largest party.
The East Kilbride, Strathaven and Lesmahagow representative was facing a selection contest in her constituency, where reports suggested that she would be ousted by SNP staffer Grant Costello.
Results of the contest were due on Thursday.
'No faith remaining'
In a statement released, Dr Cameron says her experience in the SNP Westminster group has caused a deterioration of her health over the last year.
"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."
Dr Cameron also claimed that the independence debate has cause rifts in families, including her own.
She 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."
She's since been welcomed to her new party by Scottish leader Douglas Ross and Prime Minster Rishi Sunak.
The PM said: "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."