The email was sent to Laura Murray, his daughter, who is now Labour’s head of complaints; Karie Murphy, Mr Corbyn’s chief of staff and a close friend of Mr McCluskey; and Seumas Milne, Labour’s director of communications.
Mr Murray is currently on secondment to the Leader of the Opposition from Unite, where he is chief of staff to Mr McCluskey, the general secretary.
Labour sources believe that many similar discussions have taken place on non-Labour Party email addresses, as a deliberate tactic to avoid future scrutiny.
One senior Labour source told the JC there is a fear that “a whole raft” of antisemitism cases involving Labour members has been discussed by members of Labour’s high command on email addresses belonging to Unite.
The source said: “It would appear that discussions of many Labour antisemitism cases have taken place on email addresses linked to Unite the Union.
“While some included open engagement with Labour members on email addresses linked to the party, there is obviously concern that some cases may have been discussed privately outside of Labour’s email trail among party members who also retained the use of Unite email addresses.
“From what I can ascertain, we may be talking about a whole raft, rather than a few single emails.”
Labour’s current general secretary, Jennie Formby, is among those to have previously worked at Unite, where she was political director and south east regional secretary. Mr Corbyn’s chief of staff, Karie Murphy, is also closely linked to the union.
The JC’s revelation over the use of external Unite email addresses to discuss Labour antisemitism cases could mean that the Equalities and Human Rights Commission may have to widen the scope of its investigation and demand Mr McCluskey’s union also hand over evidence relating to the commission’s inquiry.
The EHRC confirmed last weekend: “We have the power to compel the Labour party to disclose relevant information to us.”
Any attempt to destroy relevant information would be considered a criminal act.
In the email obtained by the JC, Mr Murray writes that he is “not sure” about a decision reached by Sam Matthews, the party’s head of disputes, about a Labour activist and Jewish Voice For Labour member named Max Tasker.
Evidence submitted to the party showed that Mr Tasker had also written tweets linking Isis to Israel and had accused Labour MPs linked to the moderate Progress group of “taking a bung from the Israeli government”.
Mr Tasker also wrote that Labour deputy leader Tom Watson had assured the “Zionist lobby of his unconditional support of Israel.”
Mr Murray wrote that “people disagree about the mural in a way that is not in itself antisemitic.”
He added: “I would think that investigation without suspension at this stage may be sufficient.”
It is unclear whether an investigation into Mr Tasker subsequently took place.
Peter Mason, the Jewish Labour Movement’s national secretary, confirmed that his organisation would be submitting to the EHRC over 100 witness testimonies describing alleged antisemitic behaviour in Labour.
The JLM, who are being represented by top legal firm Mischon de Reya, are likely to play a key role in the commission’s investigation. “They don’t have to consult us but, given the material we have supplied to them and anything we might continue to give them, it’s highly unlikely that we won’t be consulted,” Mr Mason said.
Labour sources said the emails referred to were during the transition period between general secretaries when former staff chose to request support from some Party staff in the Leader's Office, including Andrew Murray, in his capacity as an employee.
In relation to the allegation that Unite is involved in cases, a Labour spokesperson said: “This is baseless. Only employees, NEC and NCC members are involved in complaints.”
The Campaign Against Antisemitism have also submitted evidence with the assistance of lawyer Adam Wagner.
The EHRC is unlikely to finish its inquiry until next year.
