Supporters of the detectives under investigation, who include Detective Superintendent Ray Mallon, dubbed "Robocop" for his tough stance on lawlessness, are understood to have threatened police officers and members of the public prepared to testify in the inquiry.
Forty-six police officers arefacing a variety of allegations involving corruption and malpractice, including accusations of offering drugs for information.
The officers, most of whom are from Middlesbrough CID, face 397 complaints, including more than 240 allegations of criminal wrong-doing. Eight officers are currently suspended, including Mr Mallon, the former head of Middlesbrough CID.There is no suggestion that the accused officers have been involved in witness intimidation.
The police inquiry, known as Operation Lancet, which is being headed by Andrew Timpson, the Chief Constable of Warwickshire, is understood to have uncovered evidence of witness intimidation.
Police officers who have been prepared to testify to the inquiry are understood to have complained of being told they face a backlash by their colleagues if they speak out.
Members of the public have also claimed they have been approached by serving police officers who have tried to intimidate them into withdrawing their co-operation. Among the tactics allegedly being used are police officers visiting witnesses' homes in an attempt to suggest they are informers and thereby risk the wrath of local criminals.
A source said that there have been attempts to "turn witnesses" and intimidate them into withdrawing their evidence.
A spokesman for the Police Complaints Authority, which is overseeing the investigation, would not discuss details, but he did concede: "There is evidence of attempts to undermine the inquiry."
Operation Lancet is one of the biggest police corruption inquiries yet seen and has been running for more than a year.
The Crown Prosecution Service is understood to have received files from the inquiry team relating to allegations of CID officers supplying heroin in return for information from informers. There were also allegations that officers had threatened and beaten suspects or potential informers to obtain information. Middlesbrough CID was in the forefront of adopting the US-style tactic of zero tolerance in which all offences, however minor, were punished.
The CPS will now decide whether charges should be brought against any officers.
The inquiry is also looking to allegations that Mr Mallon passed information to the press and "alleged activity which could be construed as criminal".
In November Mr Mallon received an official warning after a second inquiry into whether he had been fiddling his expenses. He was cleared of making illegal claims. His supporters believe the second investigation was a witch hunt by officers desperate to convict him.
Last month Mr Mallon said: "I have acted honourably in the interests of the police and the public and I have acted legally."Reuse content