The Kirkcaldy MP, who quit to join Alex Salmond’s Alba Party and stood unsuccessfully on the regional list in last week’s Scottish election, said he was heartbroken to leave the party, but he “detested” what it had become.
Mr Hanvey said: “You are not allowed to have a critical mind. There is no place for debate. It is policy by diktat. It is a really uncomfortable place.”
Mr Hanvey added: “It was frustrating to see how many of the SNP cohort were far too comfy with Westminster life - that really galled me.
“What motivates me is seeing too many people in poverty - not a fancy flat in London.”
Mr Hanvey was the second SNP MP to defect to Alex Salmond’s new party, in late March, following in the footsteps of Kenny McAskill.
He said the decision to leave was “unfortunately, not that difficult.” and added: “I could not continue in a Westminster parliamentary group that was so aggressive and hostile I did not think it was good for my physical and mental health.
“It was really getting incredibly difficult to even go to work even virtually. You have to watch everything you say. If you are not loyal - and by that, they mean obedient - then you are targeted.”
Mr Hanvey’s trajectory to Westminster was far from smooth.
He was adopted as the candidate for Kirkcaldy and Cowdenbeath in 2019, and then dumped just weeks before the December election after it emerged he had used anti-Semitic language on a social media post three years earlier.,
With his name already on the ballot paper, he stood as an independent, and won.
After a period of suspension he was brought back into the SNP fold, and, in February, was made the party’s vaccine spokesman - only to be sacked within days.
By then, he said, the atmosphere was impacting on his health.
“After being sacked, I was being targeted through the media again quite purposely by the SNP - that was their choice,” he said.
“I developed a pain in my head, woke up and lost part of the visual field of my eight eye. I had damaged an optic nerve through stress.”
Mr Hanvey said matters deteriorated after the First Minister was cleared of breaching the ministerial code over her involvement in the Alex Salmond saga..
“The hostility within the group amplified to screech level,” he said.
“That was my first meeting back and I thought I can’t deal with this. It was depressing.”
Mr Hanvey said he would “absolutely not” trigger a by-election after joining Alba.
“People will judge me at the next election, but if they are expecting me to remain in the SNP, they are asking me to stay in a toxic environment that was actively damaging my health”
And Mr Hanvey believes Alba still has a key role to play in Scottish politics.
“We have active members coming across, and the energy and enthusiasm from them has been really inspirational.
“I do not think Nicola Sturgeon will secure an independence referendum, and, if she does, she has not done the groundwork to win.”
The SNP declined to comment when contacted by The Scotsman.