A new inquiry must be held into allegations about the awarding of contracts by a government agency after a previous investigation was discredited, it has been claimed.
A four-year probe into deals for the supply of road signs was strongly criticised by the Audit Office, who said it was unacceptably long, poorly planned and lacked objectivity.
It has prompted calls for a fresh inquiry into the original claims made by whistleblower David Connolly, a contractor based in Co Down.
Mr Connolly made 29 allegations about Roads Service's procedures, including claims of favouritism and tender criteria being altered.
An internal review by the Department for Regional Development concluded proper practices had been followed.
But the Audit Office said the investigation had been "seriously undermined" and there was evidence of favouritism.
East Antrim MLA Stewart Dickson, who sits on the Stormont committee which scrutinises DRD, said there was a lack of desire by officials to establish the truth.
He has called for a new investigation into Mr Connolly's allegations.
"The findings by the Audit Office are very strong and I think the claims need further investigation," he told the Belfast Telegraph.
"The Audit Office has suggested that there may have been favouritism or bias, and there is a need to find out exactly what went on.
"You cannot have that in public sector procurement."
Although the DRD investigation began in November 2005, the findings were not published until January 2010.
It concluded the process had been carried out properly.
However, the Audit Office was scathing in its criticism of DRD's investigation, saying its credibility had been weakened by a string of failings, including:
- It had not been well planned;
its independence and objectivity were weakened because evidence gathering was devolved to some officials who were implicated in the allegations;
lThe claims were not addressed in a timely and prompt manner;
- All allegations were not adequately investigated.
It concluded that there were strong indications of favouritism towards a particular firm or bias against Mr Connolly.
The Audit Office report will come before Stormont's DRD and Public Accounts committees in the coming months.
SDLP MLA John Dallat, who sits on both committees, said the report was shocking.
"I'm the longest serving member of the PAC and in all my days I have not seen worse than this," he said.
Ulster Unionist MLA Michael Copeland, who also sits on the PAC, said: "Anyone tendering for any contract, particularly those which fall within the public arena, must have a reasonable expectation of fair and equitable treatment," he said.
Mr Connolly, who brought the original complaint, said that while he felt vindicated by the Audit Office's findings, the report did not go far enough.
"It is outrageous how they [DRD] dealt with these allegations," he said. "They refused to answer those questions and I was treated with disdain and contempt for even attempting to ask those questions."
Key findings of Audit Office:
• DRD's investigation was "unacceptably long"
• It was not well planned
• Officials implicated in the allegations were asked to gather evidence in the probe
• Strong indications of favouritism to a firm or bias against the whistleblower
• Probe was unprofessional