Labour has excluded from the party a Trotskyite whose website carries a series of anti-Zionist conspiracy theories.
Gerry Downing was thrown out on Wednesday night following complaints that he had been re-admitted as a member.
He had been let back in to the party recently by its national executive committee after he appealed against being expelled last year.
A Labour spokeswoman said the decision to throw him out for a second time was made after further evidence came to light.
Mr Downing posted a tweet in 2014 referring to what he called “Hamas heroism”, appears to have questioned aspects of the Holocaust, and claimed prominent historian Ian Kershaw had a “Zionist minder”.
The website of his Socialist Fight group has published articles including one which demands that “Marxists must address the Jewish Question” and another referring to a “world Jewish-Zionist Bourgeoisie”.
It has also been reported that Mr Downing claimed the terrorists who carried out the September 11 attacks should not be condemned.
One article on the site from January, apparently written by Mr Downing, appeared to question Nazi culpability for the Holocaust and claimed that Professor Kershaw had been “dominated” by a “Zionist minder” during a lecture Mr Downing had attended.
He questioned “guilt-tripping over the Holocaust” and “Zionist money” in the rambling article.
A Board of Deputies spokesman said: “If the comments attributed to Gerry Downing about Jews are genuine, he is an obnoxious antisemite who should not be accepted in any modern political movement.”
David Cameron had challenged Mr Corbyn on the issue — first revealed on the Guido Fawkes website — during Wednesday’s Prime Minister’s Questions. He said Labour must remove Mr Downing as a member. Mr Corbyn did not respond.
Labour MP John Woodcock, an ex-Labour Friends of Israel chair, had written to Mr Corbyn urging him to expel Mr Downing. Mr Woodcock said the activist’s views were “sickening and bring our party into disrepute”.
The Labour spokeswoman said: “Following further evidence that has come to light, Gerry Downing has now been excluded from the Labour Party by the NEC panel.”