A founder member of Labour for Independence has been barred from re-joining the party to vote for Jeremy Corbyn.
Allan Grogan joined the Scottish Socialist Party (SSP) in the aftermath of 2014's No vote.
But he recently applied to become a Labour member again amid speculation that the party is heading for another leadership contest.
However, the Herald understands that his application has been blocked.
Labour's rules state it can refuse to admit members of other political parties or those who have campaigned against Labour.
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The Labour for Independence (Lfi) group claimed to have significant support in the party
But No campaigners attacked it amid accusations it was an SNP-front.
More than 100,000 people have joined Labour in recent weeks as Mr Corbyn comes under increasing pressure over his leadership in the wake of the shock Brexit vote.
Mr Grogan said that he was disappointed and criticised what he said was a "cynical" decision.
He said his decision to join the SSP has been a "mistake".
"The Labour party has always been my home. And I would have thought at the moment that Scottish Labour could do with all the help it could get," he said.
He said that many of the SNP's new members were ex-Labour, but had not been turned away.
"People change political parties all the time, even politicians cross the floor," he said.
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He added: "This sounds like a cynical move.
"There are more than 100,000 new members, with the majority supporting Jeremy Corbyn, and it looks like they are desperately trying to cut that number back as much as possible".
On his blog earlier this month he wrote: “It is clear to me that we must unite and support Jeremy Corbyn and enable him to make the case for deselection of his MP's in any forthcoming election and campaign on an anti austerity ticket, moving forward with an economic plan that will include large scale infrastructure rebuilding, an internationalist view of any negotiations with the EU and the development of a basic income ahead of the growing technological revolution.”
One of those tipped to contest the leadership Angela Eagle yesterday repeated her warning that Mr Corbyn was “losing support in the country, he needs to do the right thing and if he doesn’t I will stand against him.”
The leader of the Unite union Len McCluskey has asked MPs to give him the weekend to try to broker some way through the stand-off.
One Labour MP said that the unions would destroy the party if they backed Mr Corbyn.
Graham Jones, a former Labour whip, said that the only future for Labour was Corbyn’s resignation “whether it takes five years, ten years... whether that is at the end of a bloodbath or common sense prevails, eventually that will happen.”
He added: “We don’t want the 1980s again, we are desperate for a unity candidate, Jeremy has to walk.”
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But Mr Corbyn came out fighting saying that the party had to address the reasons behind the vote to leave the EU.
“If freedom of movement means the freedom to exploit cheap labour in a race to the bottom, it will never be accepted in any future relationship with Europe," he said.
"But the reality is that we have allies in that cause across Europe, as on many other issues that will be at the heart of the negotiations ahead."
Meanwhile, it emerged that former Labour leader Neil Kinnock told MPs earlier this week: "This is our party, I have been here for 60 years, I’m not leaving it to anybody.”
A group of MPs, led by Labour’s Chuka Umunna, have also launched Vote Leave Watch.
He said that MPs wanted to highlight pro-Leave politicians trying to “wriggle out of the promises that they made to the electorate.”
A Labour spokesman said: “We do not as a matter of course comment on individual applications for party membership”.
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