Surveys indicate that many personnel choose not to make a complaint (Picture: Crown Copyright).
Many military personnel choose not to raise grievances as they have "little faith" in the complaints system, MPs have warned.
The Defence Committee added it "seriously doubts" the system is "fit for purpose", noting the handling of cases is "unacceptably slow".
Allegations of personnel being pressured to withdraw complaints were also raised but the committee said an "absence of proper records" meant it was not possible to establish how widespread this was.
The concerns were raised in the latest report from the committee, which found neither the individual Armed Forces nor the Service Complaints Ombudsman for the Armed Forces (SCOAF) have established an effective and efficient system for handling grievances.
The MPs recommended the Ministry of Defence (MOD) should examine how to simplify, speed up and make more efficient the processes of the individual services and SCOAF.
They also suggested a reassessment of the resources required for SCOAF.
Watch: Forces News reporter Tim Cooper looks at the findings of the report
The department should work with the services to assess staffing and training requirements for complaint handling, ensuring it is a "priority" and has enough resources, they added.
The committee's latest report, titled 'Fairness without Fear: The work of the Service Complaints Ombudsman', said the "negative culture" towards complaints has "discouraged serving personnel from coming forward and making a complaint".
It said the target of resolving 90% of service complaints within 24 weeks has not been achieved.
The report went on to explain that complaints from black and minority ethnic and female personnel are "disproportionately high":
"We seriously doubt that the current service complaints system is fit for purpose."
Surveys indicate that many personnel choose not to make a complaint, often because of worries about the impact on their future careers.
"When complaints are raised, their handling is unacceptably slow," the report continued.
Julian Lewis, the Conservative MP who chairs the Defence Committee, said: "It is essential that service personnel have a fair, effective and efficient complaints system to deal with valid grievances, but the Service Complaints Ombudsman has consistently reported that this does not exist.
"The whole system risks losing credibility in the absence of a plan to streamline procedures within the services."
An MOD spokesperson said: "Everyone deserves to work in an environment where they are valued and treated with respect, and we thank the Defence Committee for their report, which echoes some of the findings in the review we commissioned into inappropriate behaviour in the Armed Forces.
"Following this review, we have already accepted a number of recommendations to overhaul and improve the service complaints system, including encouraging personnel to come forward and creating a new authority to deal with the most serious allegations."