Wellington  - New Zealand Prime Minister Helen Clark ordered a wide-ranging investigation into the Immigration Service on Monday, saying the government's confidence in its integrity had been "shattered" by allegations of bribery and corruption.

The head of the service, Mary Anne Thompson, 53, resigned last week shortly before the influential London School of Economics confirmed that she had not graduated with the doctor's degree she claimed.

Thompson was already under investigation for her role in approving residency permits for three members of her husband's extended family from the Pacific island state of Kiribati.

And it was revealed last week that 19 proven cases of serious offences, including theft, bribery and fraud, within the service's Pacific division which Thompson established, had been covered up in the last three years.

Clark announced that the country's top government watchdog, Auditor General Kevin Brady, would conduct an independent inquiry with powers to subpoena witnesses to give evidence on oath and produce relevant documents.

It was also revealed last week that the former head of the country's civil service, Michael Wintringham, had been told there were doubts about Thompson's academic qualifications when she was acting chief executive of the prime minister's department in 2004, but did nothing about it.

An obviously angry Clark told a news conference she had been "very surprised" at that revelation, adding, "For reasons that are unclear to me, Mr Wintringham didn't take the matter any further."

She said Thompson had wrongly called herself "Doctor" while working in the prime minister's department and appeared to have been "sailing through a career in the public service" on a basis of false qualifications which the service's head had ignored after it was brought to his attention.

Clark said, "I always assumed she had a PhD, because of the title she had given herself."

Thompson was regarded as one of the country's most trusted civil servants, having risen to be the highest-ranking female official in the Treasury and a senior executive in the Department of Prime Minister and Cabinet before moving to head the Immigration Service in the Department of Labour in 2004. (dpa)