A senior United Nations official has urged the International Court of Justice to rule that Malaysia acted unlawfully by allowing local lawsuits against a UN investigator.

Several Malaysian companies are suing the Malaysian UN Special Rapporteur, Param Cumaraswamy, for defamation over comments he made to a British magazine.

The UN Under-Secretary for Legal Affairs Hans Corell argued in the international court on Monday that Mr Cumaraswamy's comments were made in the course of his UN duties, and said his case related to the independent function of all UN officials.

Mr Cumaraswarmy made the offending comments while he was investigating allegations that businesses were manipulating the Malaysian judicial system.

His trial in Malaysia is due to begin in February.