Len McCluskey, Jeremy Corbyn’s ally who is standing to be re-elected as head of Unite, disciplined his leadership rival, Gerard Coyne, last year as a punishment for addressing a group of Labour MPs in parliament without permission.

The head of Unite gave Coyne a “final written warning” for speaking at an event held by Labour for the Common Good, a group founded by Chuka Umunna and Tristram Hunt, two MPs who declined to join Corbyn’s shadow cabinet.

On 7 March, two months after the event and following a disciplinary hearing, McCluskey wrote a letter marked “strictly confidential” saying he had found that Coyne was guilty of a breach of trust and was giving him a final written warning that would sit on his record for 12 months.

In the letter, headed “A serious breach of trust”, McCluskey said Labour for the Common Good was a legitimate organisation but, “given that it is the brainchild of Chuka Umunna and Tristram Hunt who chose not to serve in a Corbyn shadow cabinet, it is clear that there are obvious differences with Unite’s political vision.”