The police watchdog for England and Wales is overwhelmed, woefully under-equipped and failing to get to the truth of allegations, MPs have said.
The Independent Police Complaints Commission needs more resources and powers, the Home Affairs Select Committee report said.
IPCC chairwoman Dame Anne Owers welcomed the report, saying the body was struggling to meet expectations.
One in four officers faced complaints between 2011 and 2012.
About 30,000 officers had faced complaints, which were mostly trivial and dealt with at a local level, committee chairman Keith Vaz, told BBC Radio 4's Today programme.
"What we would like to see is the IPCC concentrate on the really serious issues. When they have dealt with serious corruption cases, 45% of the corruption cases they have investigated have ended up with the Crown Prosecution Service," he said.
The report comes as the IPCC prepares for its biggest investigation, into the aftermath of the Hillsborough disaster in 1989.
'Not yet capable'
In a scathing report, MPs said the IPCC was overloaded with appeals. Serious police corruption cases were being under-investigated while resources went on less serious complaints.
"Police officers are warranted with powers that can strip people of their liberty, their money and even their lives and it is vital that the public have confidence that those powers are not abused," said the MPs.
"We conclude that the Independent Police Complaints Commission is not yet capable of delivering the kind of powerful, objective scrutiny that is needed to inspire that confidence."
MPs said the IPCC had too many former officers among its investigators and delegated too many complaints to the forces to investigate themselves, only to overturn the conclusions in a third of appeals.
The body also lacked specialists capable of analysing crime scenes in the critical hours after an incident involving the police, they said.
"Compared with the might of the 43 police forces in England and Wales, the IPCC is woefully under-equipped and hamstrung in achieving its original objectives," said the MPs, adding that it was smaller than Scotland Yard's own internal investigations team.
The MPs say the government should provide ring-fenced funding for investigations affecting police integrity.
They also said the IPCC should be investigating the Downing Street "plebgate" affair involving police and then-Cabinet minister Andrew Mitchell.
Dame Anne said the fact the Metropolitan Police had assigned 30 of its officers to that investigation - the equivalent of a third of her whole investigative capacity - illustrated "the choices we have to make every day".
The report said the IPCC needed to be able able to interview officers under caution.
And private firms - like G4S, Capita, Mitie and Serco - involved in delivering services that would once have fallen solely to the police should fall under the IPCC's watch.
The IPCC is preparing to investigate allegations that police officers were involved in a cover-up of failings following the 96 deaths at the 1989 Hillsborough tragedy.
Parliament has passed legislation to give the body more powers for the massive inquiry and ministers are guaranteeing funding for extra investigators currently being recruited.
Dame Anne told BBC Radio 4's Today programme: "I think there's quite a lot of validation in the report, but what the committee says, and what we have been saying ourselves, we can't do enough independent investigations, we can't exercise sufficiently rigorous oversight about the way that police deal with complaints.
"We cannot do the job the public expect us to be able to do and if we are to do that job then we need to be properly resourced to do it and given the proper powers to do it."
Chief Constable Michael Cunningham, of the Association of Chief Police Officers, said the police service was keen to work with the IPCC to improve its response to police complaints, in particular to achieve speedier outcomes.
But he said developing the role of the IPCC must also not mean the police service outsourced its own responsibility to manage complaints.
"Serious allegations of misconduct must be rigorously investigated, in many other less serious cases the police service itself is the body best placed to identify and put right mistakes, learn necessary lessons and rebuild public trust," he said.
A Home Office spokesman said: "Improving police professionalism and integrity are at the cornerstone of the sweeping reforms we are making to the police force, and the IPCC has a key role to play.
"We are already working to ensure the organisation has the powers and resources it needs to manage the challenges it is currently facing and we will shortly announce a package of new measures designed to further improve the public's trust in the police."