Corbyn was a late-runner in the race for the Labour leadership. It was widely expected that one of three party moderates would win the Labour leadership: former health secretary Andy Burnham, shadow home secretary Yvette Cooper and Liz Kendall, another shadow minister.
All three of his opponents were considered mainstream Labour and enjoyed varying levels of popularity.
He does not enjoy the support of most of his lawmakers in parliament, yet says he has a mandate from the wider party membership, including many unions.
Now, however, a letter from a branch of the union Unison, written by Mike Le Cornu, chairman of the retired members' association in Colchester, warned Labour lawmakers that candidates were chosen on "strict loyal support for party policy as determined democratically by the majority of the membership."
The letter suggested that those who were not Corbyn supporters and who remained disloyal to his leadership could be deselected at the next election.
"We have to say that we find it distasteful that some of you indulged in, what can only be described as a 'Judas' congratulations to Jeremy when his overwhelming victory was announced. With the rampant behavior of some of you, we can be forgiven for concluding that maybe it's not Isis [Daesh] or Assad that you are interested to remove but Jeremy Corbyn whose success, you feel, is a threat to your careers as politicians and to the privileges you enjoy as members of the 'parliamentary club.'
"While we are not in favor of 'bullying', it should not surprise anyone that those of you in the forefront of disruptive behavior should be the recipients of severe and emotional criticism," the letter stated.
'Threats and Intimidation'
In the run-up to the vote on December 2, there were allegations of intimidation and bullying by pro-Corbyn anti-war campaigners. Former Labour Party cabinet minister Caroline Flint said: "The threats and intimidation that people have received through social media and emails to their offices and letters to their offices has no place in political debates.
"I have never received this sort of personal abuse and intolerance to any other point of view to the one that I have been expressing, and it's unprecedented. There's been more campaigning against Labour MPs and that will only serve division and disunity," she said.