Orris is due to discover the outcome of a High Court challenge on his decision to support Priti Patel following allegations of bullying Johnson.
The FDA union conducted judicial review on a decision by the prime minister last year to go against the findings of his then-adviser at the ministerial level to support the home secretary.
The judges of the High Court are likely to pronounce their verdict in the matter on Monday.
In an investigation into Ms Patel’s behaviour, published in November last year, Sir Alex Allen found that she had not always treated civil servants with “consider and respect”.
He concluded: “His attitude on occasion has amounted to behavior that can be described as bullying in terms of the effect felt by the individuals. To that extent his behavior has been in contravention of the Ministerial Code, even though It’s just unintentionally.”
Mr Johnson, the ministerial code arbiter, said the Home Secretary was “ignorant” of her influence and was “reassured” that she regretted “unintentionally upsetting the people she was working with”. .
After “weighing all the factors”, he concluded that the code had not been violated.
But at a hearing last month, lawyers for the FDA, who represent senior public servants, argued that Mr Johnson had “misinterpreted” the word “bullying” in the ministerial code when deciding whether to call Mr Patel. The behavior of the civil servants of the U.S. has violated its standards.
He alleged that he did “the wrong direction of the law” in reaching his decision.
Mr Johnson’s lawyers argued the FDA’s claim was “not justified” and there was “no error of law”.
He said the ministerial code “does not create or impose any legal duty on ministers or the prime minister”, “is not required by law” and its content “is not regulated by law”.
The code was a “political document” and “not about protecting the rights of civil servants” who still have access to “all employment law rights”, argued the prime minister’s lawyer.
Sir Alex left his role at Downing Street after Mr Johnson rejected his advice.
Following the publication of her report, Ms Patel issued an “unreserved, full apology” and said there was “no excuse” for what happened.
Lord Justice Lewis and Mrs Justice Stein are due to deliver their verdict on Monday morning.