Supporters of Jeremy Corbyn have suggested that the Jewish judge in the Labour membership case heard at the Court of Appeal may have been influenced by his "Zionist" background.
Lord Justice Jack Beatson ruled last Friday that the party had the right to block new members from voting in the leadership contest between Mr Corbyn and Owen Smith.
The decision affects around 130,000 party members, most of them thought to be backers of Mr Corbyn's effort to remain as leader.
Mr Corbyn's campaign team attacked the judge's decision as wrong "both legally and democratically".
After the ruling, messages were posted on Twitter questioning Justice Beatson's competence - because he was born in Israel.
Social media users, including known antisemites, referred to him as a "Zio".They pointed out the judge had attended Whittingehame College, a Jewish boarding school in Bournemouth.
Lord Justice Beatson has sat on the Appeal Court bench since 2013. Previously he was a High Court judge and is considered to be one of the foremost members of the judiciary.
According to International Who's Who, he was born in Haifa.
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