As the Belfast Telegraph revealed last week, a lengthy contracts probe requested by a High Court judge has ended with no charges being brought.

The outcome of the inquiry - into the axing of a company as a police supplier - has reignited controversy over whether it should have been conducted by an outside force.

But in a statement to this newspaper, the PSNI has spelt out its "rigorous" investigation into its own workforce.

The Fraud Squad probe followed a £400,000 court case payout by the force in October 2005 to Belfast-based firm NI Sheet Metal Works.

The damages related to a 2001 decision to strip the firm of a contract to provide armour plating for police cars.

The judge, Sir Liam McCollum, called for a criminal investigation and voiced suspicions that " person or persons" within the police service had "deliberately undermined" the company's delivery of the contract and "wrongfully discredited" it.

While the Public Prosecution Service (PPS) has now decided there is insufficient evidence to justify charges, two civilian employees of the PSNI remain the subject of "precautionary suspensions".

The head of NI Sheet Metal Works, Belfast DUP councillor Jim Kirkpatrick, has described the outcome of the police probe as predictable and repeated his view that it should have been handled by officers from another force.

In its first detailed statement following the decision on charges, the PSNI said: "All allegations made in a statement of complaint to police were fully investigated before a file was submitted to PPS.

"The investigation team conducted inquiries in Northern Ireland and England and enlisted the services of a number of experts in a range of disciplines to progress the investigation.

"Officers in the investigation team were under the command of a senior detective with an extensive background in fraud investigations and this criminal investigation into extremely complex issues was under the strategic supervision of Assistant Chief Constable Crime Operations.

"In addition, the investigation was advised by an independent expert from England and, with the agreement of the Chief Constable, five members of the Policing Board sat on an independent advisory group and received regular progress reports on the criminal investigation.

"This investigation has been the subject of considerable checks and scrutiny and has been found to be rigorous, professional, thorough and impartial."

Policing Board members are expected to raise the case with Chief Constable Sir Hugh Orde next month.

Assurances are likely to be sought on the adequacy of current contract-handling arrangements within the police service.

The PSNI statement added: "Procurement procedures were examined in detail and they have been improved to ensure there is a full audit trail and that procurement rules are followed.

"This process has been overseen by the National Audit Office, which has also reported to the Policing Board."

Board members may also ask for an update on the suspensions imposed on two PSNI employees in late 2005.

The fact that these staff members are still not back to work suggests that internal procedures relating to the armour plating contract saga have yet to be concluded.