The SNP has suspended MP Angus MacNeil from its membership after he launched an attack on the party’s approach to winning independence.
Mr MacNeil had already been disciplined last week after a public bust-up with the Westminster group’s chief whip about his attendance in parliament.
But after his week-long suspension expired Mr MacNeil refused to return to the party fold, saying he would not return “until it is clear that the SNP are pursuing independence”.
Now, the SNP says its national secretary believes the MP has broken the party’s member code of conduct.
His membership has been suspended while the conduct committee investigates.
An SNP spokesman said: “Angus MacNeil MP was advised by the SNP National Secretary on Wednesday that she considered him to be in breach of the party’s code of conduct by his decision to resign from the SNP Westminster parliamentary group.
“Having acknowledged this, Mr MacNeil did not take up the offer to rejoin the SNP parliamentary group and the matter was, therefore, yesterday referred to the SNP member conduct committee for consideration.”
One of the party’s longest serving members of parliament, Mr MacNeil has been an increasingly vocal critic of the leadership’s independence strategy.
In his letter saying he would not re-take the SNP whip until at least October, he accused the leadership of being “utterly clueless” and “kicking the can down the road”.
Speaking on Wednesday Mr MacNeil told the BBC he had “no plans” to join Alex Salmond’s Alba party after speculation he may join the breakaway group.
Scottish Conservative Chairman Craig Hoy MSP, said: “This is further clear-cut evidence that senior SNP politicians are continuing to fling mud at each other.
“After Angus MacNeil’s scathing criticism of the party’s leadership, they’ve now suspended one of their longest-serving MPs.
“Perhaps Mhairi Black and other SNP MPs need to look closer to home when railing against the toxicity in politics.
“The increasingly bitter infighting is clearly playing a significant role in why SNP MPs are deserting their seats, which sums up their lack of faith in Humza Yousaf’s leadership capacity to fix these internal feuds.”