SNP MP Dr Lisa Cameron has quit the party, defecting to the Conservatives.
This comes after she claimed to have faced "toxic and bullying" behaviour from party colleagues.
She is said to have quit the party just days before the Conservative Party Conference.
Cameron said she received support from Prime Minister Rishi Sunak, but heard nothing from her own party leadership.
Cameron said she received support from Prime Minister Rishi Sunak, but heard nothing from her own party leadership
PA
She announced her defection on the day she faced a selection contest for her seat of East Kilbride, Strathaven and Lesmahagow.
The contest could have seen her ousted as the SNP candidate for the constituency.
The MP for East Kilbride claimed the incidents occurred after she challenged support given to former chief whip Patrick Grady.
Grady was suspended from the Commons and forced to apologise in Parliament after he was found to have acted inappropriately towards a party staffer.
Speaking to the Daily Mail, 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.
"It is also true that I have received no contact from party leadership in the past weeks, despite members of every other main political party contacting me to offer support and compassion during what has been an extremely difficult time."
She added: "I am particularly grateful to the Prime Minister in valuing my continued contribution to Parliament as a health professional and in taking time to listen.
"It is the first time I have felt heard and shows positive, inclusive leadership in contrast to that which I have encountered in the SNP at Westminster over many years."
Responding, Sunak said: "I am delighted that Lisa Cameron has decided to join the Conservatives. She is a brave and committed constituency MP.
“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.
“I look forward to working with her on the disability issues she has championed so passionately in Parliament, and on the issues that really matter to her constituent’s in East Kilbride, Strathaven and Lesmahagow.
“Her decision is a sign that we are the party for those who will make constructive, long term decisions for a brighter future for the whole of the UK.”
Cameron said she felt "hostility" from parliamentary colleagues, accusing MPs of not engaging with her.
She first spoke out about the issue during the selection contest as the party decides who will stand as a candidate in her seat of East Kilbride.
The MP previously said that if she was not selected she could quit with immediate effect in order to trigger a by-election.
Speaking to the Scottish Daily Mail, she said: "Basically, the SNP gave me panic attacks.
"For months people wouldn't speak to me, some people when I would go into a room would just behave as though I was not there.
"I'm not saying people should have to speak to someone if they don't wish to, but it felt very much like a group bullying mentality, almost like a sort of team bully."
She took aim at the party leadership, claiming it "basically issued a directive to support the perpetrator".
Cameron said she had wanted the party to take a "victim-led approach", claiming her suggestion was "basically ignored" - after which time the "hostility" is alleged to have started.
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The MP for East Kilbride said she told SNP Wesminster whips that she was undergoing counselling as a result of the experience.
She told the Scottish Mail that she had about "12 months of counselling records that go along with this".
First Minister Humza Yousaf said the party is "in the middle of a selection contest", saying that there should be "robust debate" as part of this.
The SNP leader added: "It will rightly be up to local branches to make a decision and local members to make a decision who they want to represent them in the upcoming general election."
