The police investigators have proved that they are guided exclusively by the principles of legality and expertise, he told the news portal 24ur.com on Wednesday.

Lamberger said that police investigators "are unbiased in their work no mater who is a suspect", and condemned ungrounded criticism.

He said that the NBI had launched the investigation on the basis of a report it received in April and that the investigation had been all along guided by the Specialised State Prosecution.

"The fact that the police is one of the institutions most heavily overseen in the country is a guarantee of its legal and professional work.

"I stand behind by employees and given their professional work, I see no reason for my resignation," said Lamberger.

Lamberger came to head the NBI less than two months after the Janez Janša government was sworn in in mid-March.

He was appointed by Anton Travner, who became police boss at the new government's very first session. Travner appointed him after sacking his predecessor Darko Muženič.

Lamberger's comment comes after Interior Minister Aleš Hojs accused the police of being politically motivated, which he cited as the reason for his resignation.

Hojs said his powers were limited under the law so he had no leverage to directly influence the police work.

His statements were criticised as unfounded and detrimental to the police not only by most of the opposition parties but also police associations and trade unions.