Humza Yousaf has described Lisa Cameron's defection to the Conservatives as "the least-surprising news I’ve had as leader of the SNP."

The First Minister said the MP "should do the honourable thing" and resign her seat.

He also said the party would not fear a by-election in her East Kilbride, Strathaven and Lesmahagow constituency. 

That's despite the SNP suffering a humiliating defeat in the neighbouring Rutherglen and Hamilton West by-election last Thursday. 

READ MORE: SNP MP Lisa Cameron defects to the Conservatives

Dr Cameron stunned her colleagues on Thursday morning when she announced she was joining the Tories after eight years as an SNP MP. 

She said she had been bullied by colleagues and left requiring counselling and antidepressants.

Dr Cameron said this was because of her support for the teenage victim of Patrick Grady.

The Glasgow North MP was suspended from the Commons after he made an “unwanted sexual advance” to a teenage party staffer 19 years his junior “while under the influence of alcohol.”

Dr Cameron, a former consultant clinical psychologist, said she was "ostracised” by colleagues for asking about the victim. 

She told the Daily Mail: “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.”

Dr Cameron also said the SNP’s founding cause of Scottish independence had led to “significant division” in families like hers.

The defection comes just three days ahead of the SNP's conference in Aberdeen.

It also came just hours before a contentious internal selection battle in the South Lanarkshire seat was due to come to an end, which Dr Cameron looked almost certain to be ousted by her local party. 

READ MORE: Husband of Tory defector Lisa Cameron 'steps back' as SNP councillor

Asked about her defection, the First Minister told the PA news agency: “It’s the least-surprising news I’ve had as leader of the SNP, I must confess.

“Lisa Cameron should do the honourable thing, she should resign her seat.

“She should do the honourable thing by her constituents, who voted for an SNP MP, did not vote for a Conservative MP.”

He said he was confident the SNP could win any by-election and her decision was a “betrayal” of activists who had campaigned for her election in East Kilbride, Strathaven and Lesmahagow.

Mr Yousaf added: “To see somebody who claims to have supported Scottish independence cross the floor to the Conservative and Unionist Party betrays the fact that she probably never believed in the cause in the first place.”

However, in an interview with the Wings Over Scotland website, Dr Cameron said she had not “had a significant change of heart about independence.”

The pro-independence blog said the MP had grave misgivings about it being achieved under the current SNP leadership.

If accurate, it means she will be the first publicly pro-independence MP in the Scottish Conservative and Unionist Party. 

READ MORE: Explained: Why Lisa Cameron quit the SNP for Conservatives

Scottish Tory leader Douglas Ross said he was "delighted" to welcome Dr Cameron to the party.

“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.

“Lisa took a principled stand in supporting the victim in the Patrick Grady case, when her party took the side of the disgraced MP. For doing so, she has been shamefully and inexplicably mistreated by the SNP.

“I look forward to working with Lisa, who recognises the Conservative Party and the Prime Minister are focused on the issues that matter to people in Scotland and across the UK.”

In the 2019 election, Dr Cameron took 46.4% of the vote as an SNP candidate, with Labour winning 22.7% and the Conservatives 21.2%.

In a statement, East Kilbride SNP said the general election "cannot come soon enough."

They added: “Lisa Cameron’s attacks in the press against her own colleagues and local members of the SNP demonstrated her true character. 

"The fact her decision to join the Tories just hours before the selection results will come as no surprise. It is clear she lost faith with members and had very little support. 

"We feel sorry for the members and supporters who placed their faith and support in her. For many it will be a slap in the face that she joined the Tories.

"East Kilbride and Strathaven deserve an SNP candidate who will stand up for the local community, championing independence as the solution to getting rid of the Tories once and for all, whilst calling out the cruel and unjustifiable policies they have inflicted upon us over the past thirteen years.

"We look forward to the upcoming general election campaign, which cannot come soon enough."