Fred Leplat, who submitted the letter, insisted that each signature was genuine, and that all signatories were shown it before publication.
Mr Leplat, a member of the Finchley and Golders Green Labour Party, said: "The letter was only printed after it was shown to everyone who signed it. It was not changed afterwards, and no names were added without permission.
"It would be quite improper for me or anybody else to add names without members' knowledge, or to make changes to the letter.
"Maybe some people have had a change of heart, or perhaps they didn't read the letter properly."
A spokesman for the Barnet Labour Party later said: "The party has started a disciplinary investigation into Mr Leplat's conduct after members complained that his accusations against the two council candidates were 'grossly detrimental' to the party.
"Mr Leplat could be suspended from party membership or expelled if the complaints are upheld."
The letter defended Mr Corbyn’s anti-racist credentials and praised the party’s new General Secretary, Jennie Formby, who has issued a directive to implement the recommendations of the 2016 Chakrabarti report into antisemitism.
