Mr Rayner’s damning verdict on Ms Viner’s leadership also extended to strategy. She is currently spearheading an attempt to offload The Observer to Tortoise Media, a loss-making start-up founded by the former BBC News director James Harding.
Mr Rayner, son of the late agony aunt Claire Rayner, branded Ms Viner’s pursuit of the deal “absurd and deranged”, ahead of a crucial board vote on Monday.
In his message, he also branded the planned sale of the Observer “craven” and took aim at Ms Viner for her role in it. He said: “It represents a failure of duty of care to her staff, and crass mismanagement.”
Guardian journalists are to stage a strike for four days next month as anger about the sale boils over. On Friday, Ms Viner invited staff to meetings in her office to “hear your views and share some of my thoughts” on the deal.
The National Union of Journalists has branded the sale a “betrayal”. It has also accused Guardian bosses of “inappropriate” attempts to root out strikers after asking staff whether they intend to take part in the industrial action.
Anna Bateson, the chief executive of Guardian Media Group, has defended the sale, insisting it would ensure a sustainable future for The Observer. However, she has warned of “difficult decisions” if it does not go through.
Mr Rayner, who presents Kitchen Cabinet on BBC Radio 4 and will join The Financial Times in a few months’ time, said The Observer would “always be a part of me” after landing his first column at the newspaper in 1988 at the age of 21.
But he added: “I’m not sorry to be leaving Guardian newspapers … I have a new job and I’m very happy about it.”
A Guardian spokesman said: “We take allegations of this nature extremely seriously. The Guardian Media Group has a zero tolerance approach to antisemitism and all forms of prejudice and discrimination. We have robust processes in place. When any allegations are made, we investigate them and deal with them swiftly.
“We recognise the strength of feeling about the proposed sale of the Observer. Our priority is to support our staff and serve our readers, so that the Guardian and the Observer can continue to promote liberal journalism and thrive in a challenging media environment.”