P Lisa Cameron quit the Scottish National Party on Thursday and joined the Conservatives in a shock defection.
The East Kilbride MP has previously claimed she was a victim of “group bullying” at Westminster and suffered panic attacks as a result.
Her move is a fresh blow to the SNP under new leader Humza Yousaf following the loss to Labour in the Rutherglen and Hamilton West by-election last week.
Prime Minister Rishi 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.”
An SNP spokesperson said: “The people of East Kilbride, Strathaven and Lesmahagow will be appalled they are now represented a Conservative and Unionist MP.
“Lisa Cameron should now do the right thing and step down to allow a by-election.
“Her constituents elected an SNP MP, not a Tory, and they deserve to have the democratic opportunity to elect a hard-working SNP MP who will put the interests of Scotland first.
“On a personal basis, we wish her well.”
After Michael Shanks won the Rutherglen by-election for Labour, shadow Scottish Secretary Ian Murray tweeted on Thursday: “ While @mgshanks will be swearing into Parliament on Monday as a Labour MP, Lisa Cameron will be crossing floor from the SNP to the Tories
“The choice at the next General Election couldn’t be starker. As Labour are set to get rid the Tories the SNP are joining them.”
The East Kilbride MP’s change of party comes on the day she was facing a selection battle to be the candidate in the west of Scotland seat of East Kilbride, Strathaven and Lesmahagow.
Dr Cameron claimed she was shunned by other SNP MPs at Westminster after challenging the support given to former chief whip Patrick Grady – who was suspended from the House of Commons and apologised in Parliament after being found to have acted inappropriately towards a party staffer.
Last month, it emerged a Scottish Government minister had backed another party worker, Grant Costello, who was challenging Dr Cameron to be the next SNP candidate in her seat at the General Election.
Dr Cameron told the Scottish Daily Mail: “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 victim’s 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 praised Mr Sunak’s “positive” leadership, saying it contrasted to that of the SNP group.
Conservative MP John Lamont posted a “warm welcome” to his new colleague on Twitter, while Tory MSP Murdo Fraser said: “As the SNP continues to implode, this is another signal that ?@ScotTories are making gains at their expense.”
Scottish Conservative leader Douglas Ross said on Twitter: “It’s great to have Lisa Cameron on board.
“The @ScotTories will stand up for everyone who has been forgotten by the SNP to get the focus on to Scotland’s real priorities.”
