There was "no realistic prospect" of convicting eight police officers involved in the Lynette White murder investigation, the High Court has heard.
The former officers have brought civil action against South Wales Police after being cleared of alleged corruption.
The corruption trial collapsed in 2011.
At the Cardiff court on Tuesday, senior Crown Prosecution Service (CPS) lawyer Gaon Hart said he stood by his advice that conviction was unrealistic.
Former officers Graham Mouncher, Thomas Page, Richard Powell, John Seaford, Michael Daniels, Peter Greenwood, Paul Jennings and Paul Stephen are suing the force for misfeasance in public office and false imprisonment
Mr Hart told the court how the relationship between him and the senior investigating officer and the Crown's lead counsel had deteriorated.
He said he had been called perverse and verbose, that he had been patronised and had his intelligence insulted.
Asked by Anthony Metzer QC, representing some of the officers, whether he was suggesting that the CPS should "not embark on a prosecution that was flawed" he answered: "Yes".
He was asked about his concerns regarding the way officers conducted interviews in 1988 and whether it would be illegal now, or whether it was illegal then.
"I was concerned that if the officers said 'that's how we did it in those days' it would make proving conspiracy more difficult," Mr Hart said.
"Because if all of them independently had taken illegal action, that's not a conspiracy."
The case is continuing.