The professional standards department of Greater Manchester Police (GMP) is "corrupt", a former Police Federation expert has told the BBC.
Aidan Kielty, who retired two years ago, said his concerns about the GMP unit were not properly investigated.
Mr Kielty spoke after a sacked police officer claimed he was the victim of "corrupt practice" within the force.
GMP said former Ch Insp John Buttress had "fallen below" the force's accepted honesty standards.
Before his retirement, Mr Kielty was responsible for the Criminal Investigations Departments for the Police Federation, which represents rank-and-file officers.
He was also responsible for assisting officers accused of criminal or misconduct matters.
He said he became dismayed at practices which he considered to be "corrupt, criminal, underhand and contrary to the police regulations".
His complaints about the department include concerns about the case of an officer who was dismissed for striking a prisoner without justification.
A spokesman for GMP said those concerns had been "considered and reviewed at the time".
Mr Kielty told the BBC: "From my experience, in my opinion, Greater Manchester Police Professional Standards Department would regularly lie, cheat, change witness statements and bully people into changing witness statements.
"They would prosecute an officer rather than investigate an officer. And by that I mean they would decide at the earliest opportunity whether an officer was innocent or guilty. And then work towards that end game."
Mr Kielty claims the branch was "not trusted" by the majority of officers "because they don't follow the rules".
He added: "They breach police regulations and the law in respects of obtaining evidence. They do it on a regular basis."
He said the practice appears to continue.
On Monday, Ch Insp John Buttress said he believed there was corrupt police practice in targeting him.
He was cleared in January by a crown court jury in Liverpool of mortgage fraud but, under a lower burden of proof, was brought before a police disciplinary panel.
A spokesman for GMP said: "We have been working on a range of developments to the way in which we conduct internal investigations.
"This has included working with the Police and Crime Commissioner to introduce an ombudsman.
"It is understandable that staff who have the difficult job of investigating colleagues sometimes find themselves facing allegations from those they investigate."