WASHINGTON (AFP) -- The office of Iraq's Prime Minister Nuri al-Maliki has used a Saddam Hussein-era law to halt dozens of corruption-related probes of government ministries, a U.S. auditor's report said.

The report by the special inspector general for Iraq, Stuart Bowen, said Iraq's Commission on Public Integrity (CPI) estimates the cost of corruption at around five billion dollars a year. The real cost of corruption is difficult to measure, the report added, warning that "opportunities for corruption have increased."

"The Prime Minister's Office has ordered CPI not to refer to any investigative court any case involving a minister or former minister without prior approval of the Prime Minister," Bowen's quarterly report to Congress said.

In doing so, the prime minister has invoked a law originally enacted in 1971 that requires that the minister of an affected agency's give permission for a corruption case to go to trial.

The law was originally intended to be applied after an investigation by an investigative judge, but is now being used to stop investigations before an investigative judge has decided whether to bring a case to trial, the report said.

The law, Article 136B of the Iraqi Criminal Procedure Code, had been suspended by the U.S.-led Coalition Provisional Authority that ran Iraq before the restoration of sovereignty. It was reinstated by the prime minister, the report said. A review of corruption-related cases by the Iraq Reconstruction Management Office "showed that ministers have stopped prosecution and investigations on 48 cases involving 102 individuals under Article 136B." The report added that some observers believe that the law was "a necessary check to an anticorruption effort that has become politicized." However, it said the head of the US embassy's Office of Accountability and Transparency believes that the ministers' power to halt investigations, as well as the lack of security for investigators, "are hindering efforts to combat corruption in Iraq."