THE SNP's longest-serving MP has quit the party's frontbench with a stinging attack on the Westminster group's new leader. 

In a scathing resignation letter, Pete Wishart said he was "bemused" at why Stephen Flynn thought it necessary to oust Ian Blackford. 

He also said the Aberdeen South MP had not discussed his plans for the group when "canvassing opinion for a leadership challenge" or during the weekend's leadership contest.

READ MORE: Blackford: I could have won if I’d put myself forward in SNP race

His resignation comes a day after Mr Flynn's ally, Stuart Hosie, said reports of splits in the Westminster group were a “complete fiction”, and that he had “no idea where these stories have come from”.

The Herald:

Mr Wishart had been the party's Department for Environment, Food and Rural Affairs (Defra) spokesperson since September. 

It is likely he jumped before he was pushed. Last week, sources close to Mr Flynn told press he would be making sweeping changes to the frontbench with very few MPs retaining their jobs.

READ MORE: Flynn attacks Labour and Tories in PMQs debut as SNP Commons leader

Last night, he sacked Midlothian MP Owen Thompson as the party's chief whip, with the job going to West Dunbartonshire's Martin Docherty Hughes.

In an email to his MPs, Mr Flynn said this decision "precedes a small reshuffle of the front bench, the creation of a political cabinet and a realignment and refocus of our existing policy groups."

In his letter to Flynn, Mr Wishart - who backed Alison Thewliss in the party's leadership contest - said: “In my 21 years in parliament, I have served as spokesperson in a variety of front bench roles including as our shadow leader of the house and chief whip.

"I have never had the opportunity to experience the delights of the backbench and I look forward to serving your leadership from there."

One of the dividing lines between Mr Flynn and Ms Thewliss in the contest was over the Westminster group's closeness to the leadership in Holyrood.

Mr Blackford lost the support of a number of his MPs after Edinburgh told them to abstain on a vote on low level benefit uprating, rather than oppose it completely.

Ms Thewliss was seen as a close ally of Nicola Sturgeon, while Mr Flynn was perceived as more independent of party HQ. 

In his letter, Mr Wishart said: "I note that you hope to bring positive change to the way the group is run and that you wish to reset the relationship with colleagues in Government and in the Scottish Parliament.

"The latter, as I'm sure you are aware, is extremely important for the general culture within the party and I wish you well in these endeavours.

"The Westminster group is unique in the respect that we are a party of power in a parliament, in which that power can never be exercised.

"Many of us carry the scars following the many attempts to manage that unfortunate tension over the course of the years. 

"I remain bemused as to the reasons why you felt it was necessary to seek a change in our leadership, particularly when we see yesterday's opinion poll, which shows support for independence at a near all-time high and support for the SNP at Westminster at an unprecedented 51 per cent."

READ MORE: Westminster Sketch: As 'giant' Blackford departs, Flynn the skin gets his Doc Martens on

Mr Wishart said a change in leadership was usually because of failure, while the SNP were "looking only at sustained and growing success as a movement and party."

He added: "I am sure that this is something that will become apparent to me during the course of your leadership.

"I also look forward to learning at first-hand what you hope to do differently in the day-to-day management of the group.

"Lastly, I wish to assure you that you have my full support and I remain at your service if you do wish to make use of my experience in the Westminster group.

"We are always better when we work together, fully support our Government and stand side by side with all colleagues regardless in which Parliament they serve."

Responding to Mr Wishart's resignation, Mr Flynn said his "door remains open."

He tweeted: "Pete, thank you for your exemplary service for the SNP on the front bench in Westminster over so many years.

"As ever, my door remains open and I look forward to working with you as we support colleagues in Holyrood, hold the Tories to account, and work to deliver independence."

Yesterday, Mr Hosie, the Dundee East MP, told BBC Radio Scotland that the party was “absolutely united” under the “fantastic leadership of Nicola Sturgeon”.

He said: “With Stephen Flynn at the helm in Westminster I think we'll see closer working [between MPs and MSPs] than we've ever done."

Scottish Tory Chairman Craig Hoy said the SNP were "fighting like ferrets in a sack."

He said: "His scathing attack on Stephen Flynn for bringing down Ian Blackford flies in the face of Stewart Hosie trying to pretend all is sweetness and light within the party, and is a clear sign of Nicola Sturgeon’s waning influence on SNP MPs.

"Policy and personal disagreements are emerging all the time as the party’s reputation for iron discipline under Nicola Sturgeon disintegrates.”