The row follows hot on the heels of similar claims made in defence of Home Secretary Suella Braverman after a leak about her and a speeding ticket seemingly done with the intention of forcing her out.
Mr Tierney was involved in investigating Partygate and it has been claimed by a former minister that he would have been involved in any discussions to refer Mr Johnson to the police as Director of Propriety and Ethics.
But the letter in the court case raises serious questions about his fitness for the role.
The ex-official, who recently resigned because of the culture in the civil service, wrote in a section of his statement entitled “political bias”: “I personally learned not to say anything remotely positive about Brexit.
“In my first one-on-one meeting with Darren Tierney, I referenced the potential trade opportunities…post Brexit.
“Darren Tierney took a visceral reaction to that suggestion, and I feel I was blackballed by him after that.
“I left Whitehall and took my current job at a lower Civil Service grade from my last job in an effort to get away from Whitehall.”
The former official went on: “I am disappointed that Darren Tierney has been appointed Director General for Propriety and Ethics at the Cabinet Office upon the recommendation.
“Darren Tierney replaced Helen McNamara as the Director General for Propriety and Ethics. Helen McNamara was investigated by the Metropolitan Police for violating the Covid laws that she authored and was subsequently fined.
“The British public deserves better civil servants in leadership.
“There is no point in complaining about anything within internal Civil Service structures with Darren Tierney in that post.”
The claim follows other concerns about the political bias and affiliations of civil servants in the Cabinet Office.
Recently, Ms Gray, who authored the report into Partygate, caused outrage by resigning from her position to go to work for Labour leader Sir Keir Starmer.
Questions were already being asked about senior civil servants in the Cabinet Office being married to journalists who broke the Partygate stories and Labour’s shadow Chancellor Rachel Reeves.
The statement was written as a letter initially to a member of the House of Lords in support of the case that has been brought by a different ex-Whitehall official who has claimed he was framed for leaking information about former US Ambassador Lord Darroch because he was perceived as “a rightwing Brexiteer.”
The former official was dragged from his bed while recovering from cancer surgery by 14 counter terrorism police, subjected to months of investigation and as a result was made homeless and attempted suicide.
The Cabinet Office has declined to comment.
However, a source said both the complainant in the court case and witness “have clear grievances” and the statement is in the context of a court case.
The source went on: “No such complaints have ever been raised regarding Mr Tierney’s conduct.
“As Director General for Global Trade and Investment, Mr Tierney worked to support the UK’s post-Brexit trade relations, including helping secure trade agreements with countries like Australia and progressing the UK’s entry into the Trans Pacific Partnership trade agreement.”
The Cabinet Office declined to answer questions on whether Mr Tierney was involved in the decision to refer Mr Johnson to the police.
