The Government is under pressure to provide further funds to the police watchdog after a damning report found it lacks the resources needed to get to the truth.
The Independent Police Complaints Commission (IPCC), which is investigating the Hillsborough disaster in the UK's biggest ever inquiry into police misconduct, is "woefully under-equipped and hamstrung", an influential group of MPs found.
The report from the Home Affairs Select Committee said around 200 police officers retire or resign each year to avoid disciplinary hearings.
IPCC chairwoman Dame Anne Owers said that without further resources, the body will struggle to meet the expectations of complainants and bereaved families.
The Home Office said it is already working to ensure the IPCC has the powers and resources it needs and is to shortly reveal new measures to improve trust in the police.
Committee chairman Keith Vaz said: "When public trust in the police is tested by complaints of negligence, misconduct and corruption, a strong watchdog is vital to get to the truth - but the IPCC leaves the public frustrated and faithless."
He added: "Nearly a quarter of officers were subject to a complaint last year. Many were trivial, but some were extremely serious, involving deaths in custody or corruption. It is an insult to all concerned to do no more than scratch the surface of these alleged abuses. The IPCC investigated just a handful and often arrived at the scene late, when the trail had gone cold. The commission is on the brink of letting grave misconduct go uninvestigated."
A total of 31,771 officers - one in four - were subject to a complaint during 2011/12 and when appeals were made against the way forces handled a complaint, the IPCC found the police were wrong in one in three cases.
The watchdog should have a statutory power to force implementation of its recommendations and in the most serious cases it should instigate a "year on review" to ensure its recommendations are properly carried out, the committee 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."