An SNP MP has accused the party's Westminster chief whip of "bullying" after the pair clashed in the House of Commons on Monday.
Western Isles MP Angus MacNeil made the claims against Brendan O'Hara, who represents Argyll and Bute, after a public falling out between the pair in the division lobbies.
Disciplinary notices sent by O'Hara to MacNeil were also claimed to have been thrown across the floor. The notices were about MacNeil's attendance record.
MacNeil then referred to the incident on Twitter, claiming that O'Hara is a bully.
He said: " "General advice - Always stand up to bullies - [especially] any abusing their positions - particularly those who scurry around shouting to anyone listening - 'I'll get him, I'll get him, I WILL get him! Mark my words'.
"These are people, who from school, we should get taught to face down."
Online blog Guido Fawkes reported that MacNeil shouted: "You're a small wee man!" several times at O'Hara before throwing the letters at him and storming off.
MacNeil, seen as a close ally of Alba leader and former First Minister Alex Salmond, has been a harsh critic of the SNP in recent months. He was against the recently dropped Highly Protected Marine Areas policy and is vocal on Twitter over his support for using elections as de facto referendums.
He has represented the Western Isles since 2005, making him the joint second-longest serving SNP MP.
The clash with O'Hara is believed to have been exclusively about his attendance in parliament.
The Times reported that the tweet's reference to "I'll get him" was about MacNeil not being in the chamber while he was campaigning to be chair of the net zero committee.
O'Hara took over from Martin Docherty-Hughes as SNP chief whip after only six weeks in the role.
The Sunday Mail reported that Docherty-Hughes was forced to step down after he wrongly attempted to out another MP as a mole.
O'Hara and the SNP were contacted for comment.
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