The Commissioner for Standards in Public Life has found that an official of the Office of the Prime Minister, Josef Caruana, breached ethics last December when he disclosed the ID numbers of signatories of a petition calling for Joseph Muscat to step down.
The complaint was made by independent election candidate Arnold Cassola.
The petition was circulated among university academics. The ID numbers of 70 signatories were revealed in a Facebook post. They were later removed.
The standards commissioner said that Mr Caruana, as a person of trust employed in the Office of the Prime Minister, had a responsibility that was higher than that of a private citizen.
"Publication of the ID numbers by a person in such a post could not but be seen as a form of intimidation, even if that may not have been what Mr Caruana intended," the commissioner said.
That the post was uploaded a day before the petition was closed could also be seen as a message to others who were considering whether the sign.
The commissioner said that the fact that the details had been removed and the finding of guilt consisted enough of a punishment.
The commissioner said he could not investigate other complaints made by Cassola against former chief of staff Keith Schembri and OPM official Sandro Craus because the alleged shortcomings were made before the Office of the commissioner was set up.
Cassola had complained that Schembri allegedly created a criminal association and Craus gave phantom jobs to unqualified people in the public service.