By Francois Murphy

VIENNA (Reuters) -Iran's acquisition of confidential U.N. nuclear watchdog documents is a 'bad' step that goes against the spirit of cooperation that should exist between the agency and Tehran, its chief Rafael Grossi said on Monday.

The International Atomic Energy Agency said in a confidential report on Iran to member states on May 31 seen by Reuters that it had "conclusive evidence of highly confidential documents belonging to the Agency having been actively collected and analysed by Iran".

The report said that "raises serious concerns regarding Iran's spirit of collaboration" and could undermine the IAEA's work in Iran, but Tehran said in a statement to member states last week that the accusation in the report was "slanderous" and had been made "without presenting any substantiated proof or document".

The IAEA's 35-nation Board of Governors is holding a quarterly meeting this week. The United States, Britain, France and Germany plan to propose a resolution for the board to adopt that would declare Iran in breach of its non-proliferation obligations over other failings outlined in the report.

"Here, unfortunately, and this dates to a few years ago ... we could determine with all clarity that documents that belong to the agency were in the hands of Iranian authorities, which is bad," Grossi told a press conference. "We believe that an action like this is not compatible with the spirit of cooperation."

Asked about the nature of the documents and whether they were originally Iranian ones that had been seized by Israel and supplied to the agency, Grossi said: "No. We received documents from member states, and also we have our own assessments on documents, on equipment, etc."

(Reporting by Francois Murphy, Editing by William Maclean)