The agreement with the Security Service is after months of negotiations about the procedures for getting secret material his office used to get automatically.

The memorandum of understanding signed by the policing watchdog, MI5 and the PSNI became necessary when the Security Service took over anti-terrorist intelligence last October.

The shift led to MI5 building a £20m base in Palace Barracks, Holywood, and recruiting agents locally. The building is also a back-up headquarters after London.

Handing MI5 responsibility for national security matters also moved many intelligence documents out of the Ombudsman's reach. The PSNI had been legally obliged to show the Ombudsman's investigators any intelligence material relevant to their inquiries into alleged police wrongdoing until the Government refused to extend that obligation to MI5.

Sinn Fein and the SDLP tried and failed during the St Andrews negotiations in 2006 to get legal guarantees from MI5 and Mr Hutchinson's predecessor, Nuala O'Loan, left office last year without signing a similar agreement.

After signing the memorandum, Mr Hutchinson said he is aware of concerns investigators could be misled or refused access and said MI5 has routinely cooperated with his office and that it would be hard to hide material.

"In practical terms it is very difficult to deny the existence of material. Our published reports show this: often there are supplementary and corroborative materials to indicate existence of documents, etc," he said.

The Ombudsman said that the vast majority of cases do not require access to MI5 documents.

"Whilst this formal Memorandum of Understanding has only just been signed, we have had informal arrangements in place for several years to get information from the Security Service and my investigators have never encountered any insurmountable problem," he said, noting the agreement assumed "that cooperation with the Security Services will continue in the positive atmosphere already evidenced".

"Let me be clear on my position. Although I have no legal authority over the Security Service, it has to date given this Office the utmost cooperation, often providing very extensive and detailed information," Mr Hutchinson said.

"In the unlikely event that my Office would be refused, there are a number of avenues, including Parliament, where I could challenge such a refusal," he added.