The European Ombudsman has found that the EU executive under President Jean-Claude Juncker failed to follow procedures in appointing his chief aide to run the EU civil service and so undermined public trust.
Publishing the findings of her investigation into the February appointment of Martin Selmayr as secretary-general of the European Commission, Ombudsman Emily O'Reilly said on Tuesday that four instances of maladministration resulted from not following rules correctly, "either in letter or in sprit".
At issue were secrecy around the impending retirement of the outgoing secretary-general, an "artificial urgency" to fill the post that went unchallenged by Juncker's 27 fellow commissioners from member states and Selmayr's appointment as a deputy to his predecessor just minutes before being promoted to the top job.
The Ombudsman also criticised the Commission for "defensive, evasive and at times combative" responses to media questions
The Ombudsman also criticised the Commission for "defensive, evasive and at times combative" responses to media questions on the arcane manoeuvres around the publication of vacancies and selection of candidates which preceded an appointment that Juncker and his fellow commissioners were always free to make.
And she faulted other commissioners for failing to query why Juncker was rushing through a new job for his chief of staff.
The findings, accompanied by a recommendation to review the procedure for the appointment of the Commission's top official, does not annul the appointment of Selmayr, whose combative style has won the German both admirers and many enemies in Brussels.
But it reinforces damage to the EU executive's image after the European Parliament accused Juncker of a "coup-like action" just as backers of stronger EU institutions gear up to fight eurosceptics in elections next May to the EU legislature that will be held in the wake of Britain's exit.
"Our inquiry ... shows the precise steps the Commission took in order to make the appointment process appear normal," the Ombudsman said in a statement.
"All of this risked jeopardising the hard won record of high EU administrative standards and consequently, the public trust."
Juncker, a former premier of Luxembourg, will retire late next year and government leaders and Parliament will oversee the appointment of a new Commission for the next five-year mandate.
As a civil servant, Selmayr would be expected to retain his role running the 30,000-strong administration in Brussels during a transition but could be replaced under Juncker's successor.