The private intervention comes at a fractious time for the party, as the leadership agonises over what its tactics should be if Theresa May loses a vote on the Brexit deal in parliament.

McCluskey is an influential figure in the party and several of Corbyn’s most trusted staff are former Unite members and close to the union leader, including the party’s general secretary, Jennie Formby, the adviser Andrew Murray and the chief of staff, Karie Murphy.