Neil Findlay, who is running Mr Corbyn’s campaign in Scotland, has urged others on the far left of Scottish politics to “put aside differences for a short period” to become registered members for £3. He has insisted he is only trying to win back Labour voters.
However acting Labour leader Harriet Harman has now come under pressure to suspend the contest amid concerns it is being warped by a wave of new members who want to push forward a hard-left agenda.
The concerns come as the number of full Labour members is expected to be 66,000 higher than on 8 May, by the cut-off point of 12 August.
Another 22,000 people are likely to take advantage of new rules introduced by Ed Miliband that allow them to pay £3 to become “registered supporters”. Only around 30 applications for that category are believed to have been rejected, most from Tories.
Some 55,000 extra “affiliate” members are also being signed up by unions, with the biggest union Unite actively encouraging members to back Mr Corbyn.
One senior Scottish Labour figure told The Scotsman that he had seen new members come in who were known left-wing extremists and had previously professed to “hate Labour”. The row has exploded as Mr Corbyn dodged questions during a broadcast interview about whether he himself is a Marxist.
He said: “That is a very interesting question actually. I haven’t thought about that for a long time.”
Mr Corbyn said the growing Labour roster was mainly due to “lots of young people who were hitherto not very excited by politics coming in for the first time.”
Meanwhile, Mr Findlay insisted that his comments asking left-wingers to put aside their differences “for a short time” were aimed at bringing disaffected voters back to Labour. He said: “During the referendum and the general election it was clear that a very significant number of Labour voters, probably lifelong Labour voters, went over to the Yes side and supported the SNP in the election.
“The Labour Party has to get these people back if we are to win again in Scotland. I have no doubt many of these people were opposed to austerity and the renewal of Trident. This leadership election is a massive opportunity to bring back these Labour voters,”
His claim was dismissed by others in rival leadership camps.
One senior figure said: “Findlay is at it. He said ‘short time’. He knows that he is trying to influence this contest with people who do not have Labour values.”
Two senior Labour backbenchers have called for the contest to be suspended.
Bassetlaw MP John Mann said the situation was “totally out of control. It should be halted”.
“It is becoming a farce with long-standing members ... in danger of getting trumped by people who have opposed the Labour Party and want to break it up, expressly want to break it up - some of it is the militant tendency types coming back in.”
Blackley and Broughton MP Graham Stringer added: “I am worried that people who do not have the interests of the Labour Party at heart are joining the Labour Party.”
However, shadow health secretary Andy Burnham, who had been the favourite until shock polls put Mr Corbyn ahead by a sizeable margin, branded demands for a pause “unhelpful”.
He said: “I’m comfortable with people joining the Labour Party. There are processes in place to check if somebody is joining for the wrong reasons.”