Mhairi Black, the SNP's deputy leader in the Commons will not stand as an MP at the next general election.

The 28-year-old said she was"tired" of "toxic" Westminster, describing it as a "horrible place to be."

She also said the threats and social media abuse had left her loved ones in a "constant state of anxiety" over her safety.

First Minister Humza Yousaf paid tribute to the MP for Paisley and Renfrewshire South, describing her as a trailblazer. 

Speaking to LBC's The News Agents podcast, Ms Black was asked why she was standing down. She said:"Honestly, because I'm tired, is a big part of it. And the thing that makes me tired is Westminster.

"I think it is one of the most unhealthy workplaces that you could ever be in. It's a toxic environment.

"Just the entire design of the place and how it functions is just the opposite of everything that I find comfortable."

Ms Black, who was 20 when she was elected, said Westminster was "definitely a poisonous place."

"Whether that's because of what folk can get away with in it or the number of personal motivations and folk having ulterior motives for things, and it's just not a nice place to be in," she added.

The MP is the sixth member from the SNP benches to announce that they would not contest the next general election, widely expected next year. 

Peter Grant, Douglas Chapman, Ian Blackford, Stewart Hosie, and Angela Crawley have all said they will leave Westminster.

READ MORE: Stewart Hosie fifth SNP MP to step down at next Westminster election

Ms Black’s victory against Douglas Alexander was one of the most unexpected results in the 2015 election. She overturned the Labour frontbencher’s majority of 16,614, and has held on to the seat at the subsequent two votes. 

The Herald: Mhairi Black reveals dad's white lies meant she was "last to know" about sensational election win

She became the youngest MP in centuries and was quickly touted as a possible future leader of the SNP. 

Ironically, as she leaves Westminster, Mr Alexander is making a bid to return, contesting East Lothian for Labour.

During the interview, Ms Black said she first decided to stand down on after winning her seat in 2019. 

She was asked if her dislike of Westminster meant she could not trust her colleagues. Ms Black replied: ‘No, of course, I work very closely with colleagues. 

“But I suppose I'm talking more about how it's difficult to know if somebody is, certainly from other parties, is talking to you, because there's a genuine relationship there, or whether they're looking for opportunities, so you can never really switch off when you're in Westminster. 

“And also, given the unsociable hours that Westminster works as well, it feels like you're spending a lot of your life there. 

“And in the run up to the next election, I've realized, that will be almost 10 years that I'll have been elected. So, a third of my life I've spent in Westminster, which gives me the ick.”

READ MORE: SNP MP admits Yousaf's independence plan will 'probably not' work

Asked if she thought that part of the problem was because she was so young and that the job had eaten up so much of her twenties, Ms Black replied: "I don't regret spending any of the time doing what I've done."

She added: "I'm fighting for what I believe in and doing so in a way that I have had the encouragement of others to push me forward and do it.

"But I actually think the fact that I'm younger, is partly why I suppose I see everything that's wrong so starkly. 

"I can understand why people get absorbed into the world of Westminster, how folk can spend forty years working there, because it's a world unto itself. It's got its own culture, its own history and everything. Which is just still alien to me." 

Ms Black said the only way to change that culture was by people like her "going in and pointing out all these things."

Further explaining her decision, the MP said she was embarking on married life and that her parents were getting older.

"And I see it in their faces when they come around to visit and they see the panic alarms and the bomb bags that are left and they see the threats that I get sent and stuff.

"It's a pretty stark reminder that their daughter's not in a normal job, so to speak. So, of course, I'm telling people how toxic the place is, so that it can be changed. And I just suppose I feel like I've done my shift for now.’

Former first minister Nicola Sturgeon said she hoped Ms Black would one day make the move to Holyrood. 

"Both gutted by and entirely understanding of this. Her reasons resonate. But what a loss of a unique talent, not just to @theSNP but to politics generally," she tweeted.

"I only hope it’s temporary. The world needs more Mhairi Blacks in politics, not fewer. I hope we will see her in @ScotParl"

Stephen Flynn, the party's leader at Westminster, who ran for the top job with Ms Black as his running mate, tweeted simply that his deputy was "in a class of her own."

READ MORE: Greens to snubs King for Scots coronation republican rally

First Minister Humza Yousaf said Ms Black would continue to make an "immense contribution" to the push for Scottish independence. 

In a statement, he said: “It is difficult to overstate the impact that Mhairi Black has had on Scottish and UK politics since her election in 2015 as the youngest MP ever, and more recently as Deputy Leader of the SNP at Westminster.

“She has been a trailblazer - a passionate supporter of independence, equality, social justice, and simply of trying to make life better for her constituents and the wider Scottish public.

"She has also served as a role model for young people, especially women, with an interest or a desire to get involved in politics.

“I know that Mhairi has been critical of the toxic, hostile environment of Westminster, which serves as an important wake-up call to those who are determined to safeguard our democracy.

"The case for modernising our political system is stark. We must make sure it works for everyone, so we don’t deter people from standing for election or speaking out for what they believe in.

“Despite choosing not to stand again as an MP, I know Mhairi will continue to make an immense contribution to the cause of independence, and I look forward to working with her in advancing our cause.”

Tory MSP Craig Hoy said Ms Black’s decision to step down was "yet another damning verdict from a senior SNP MP on the failing leadership of both Humza Yousaf and Stephen Flynn."

He added: “As much as the Depute Leader tries to blame Westminster, in typical SNP fashion, the public won’t be fooled. Mhairi Black knows chaos is engulfing her party, which is why they are fighting like Nats in a sack.

“It speaks volumes about how bitter those feuds have become that Mhairi Black has thrown in the towel, just a few months after agreeing to become deputy leader, and decided not to fight another election despite not yet turning 30.

“It is the Scottish public who are paying the price for the SNP’s infighting and civil war with the party completely distracted from focusing on Scotland’s real priorities such as the cost-of-living crisis and fixing our NHS.”