Labour's National Executive Committee (NEC) has decided to ban new members of the party from voting in the upcoming Labour leadership election.
The meeting of the party's ruling body decided by a margin of 18-14 that as the incumbent, leader Jeremy Corbyn had an automatic place on the ballot and did not need to gather 51 nominations.
But it also decided that people who had been party members for less than six months could not vote.
It will also cost £25 instead of £3 to sign up as a Labour supporter - and anyone wishing to sign up only has two days to do so.
Mr Corbyn's supporters - who had reacted with joy at the news that he was on the ballot - quickly turned on the NEC accusing it of "social exclusion" and signing up new members under false pretences.
A 38Degrees petition set up immediately after decision was announced calling for a reversal has already gathered over 2,000 signatures.
The petition, which was set up by Tim Longson, said: "We believe that as this will disqualify up to 20% of the whole Labour party membership (around 100,000 members), and that this is unacceptable and misrepresentational (sic) of the democratic Labour membership will.
"Failure to meet with this very reasonable request of such a huge body of members may possibly suggest that the NEC leadership is, in fact, deliberately trying to block the additional votes of the these 20%, indicating some bias.
"Further, in light of the fact that has been strongly suggested that the vast majority of these new full Labour party members are backing Jeremy Corbyn, and would vote for an NEC team who would also lean that way, it would suggest this bias is directed against Jeremy Corbyn's leadership."
Labour has gained 100,000 new members since the EU referendum on 23 June with may joining with the express purpose of voting for Mr Corbyn. Its total membership is now reportedly around the half a million - more than the 405,000 it reached during the high point of Tony Blair's premiership.
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