The chairman of Pieter Omtzigt’s Nieuw Sociaal Contract party has stepped down over a work-related dispute dating back 19 years.

Hein Pieper, a former Christian Democrat (CDA) MP and co-founder of NSC, resigned following an independent investigation by a professor of ethics.

Pieper was the subject of a complaint by a colleague when he was a director of VKMO, an association of Catholic social organisations.

NRC and De Telegraaf reported that the employee was made redundant shortly after he had caught Pieper using work computers to view pornography.

Pieper did not confirm the details of the complaint, but said he was resigning as party chairman for “personal reasons”.

Omtzigt had commissioned Rob van Eijbergen, professor of integrity at the Vrije Universiteit in Amsterdam, to investigate the reports after the complainant contacted the party, but Pieper resigned before his report was finished.

NSC spokesman Kilian Wawoe told the Telegraaf: “We are very grateful for everything that he has done as co-founder of our party”, adding that Pieper would still be “involved in the future of NSC”.

It is the second resignation of a senior figure in NSC since Omtzigt launched his party at the beginning of September. Three weeks ago Onno Aerden resigned as Omtzigt’s spokesman within hours of being appointed after tweets resurfaced in which he described the farmers’ party BBB as a “tumour” on Dutch democracy.