A "disproportionate" number of female and ethnic minority personnel in the armed forces file internal complaints, MPs have said.
A Commons Defence Committee report said the number of complaints, which often allege discrimination, bullying and harassment, was a "serious concern".
Some opted not to raise grievances as they have "little faith" in the complaints system, the report added.
The Ministry of Defence said it would "overhaul" the complaints process.
The report published on Wednesday said there was "no clear data" to suggest why, for the third consecutive year, female and black, Asian and minority ethnic (BAME) staff in the Army, the RAF and the Navy were "overrepresented" in the armed forces complaints system.
Women make up 11% of the workforce in the armed forces but made 23% of complaints in 2018, the report said.
In 43% of cases, concerns raised by women related to bullying, harassment and discrimination - compared with 20% of complaints by men.
People from BAME backgrounds make up 7% of the workforce but made 13% of complaints, it added.
For these people 39% of complaints concerned bullying, harassment and discrimination, compared with 24% for white members of staff.
The report said it was "unacceptable" that the Service Complaints Ombudsman for the Armed Forces (SCOAF) - which aims to provide independent oversight of the complaints process - had not been sent results of internal reviews into the system.
"This sends the wrong message about valuing BAME and female personnel, with a potentially negative impact on their retention and future recruitment," it added.
The committee also drew attention to the "unacceptably slow" handling of cases and said the SCOAF, the Army, the RAF and the Navy had all failed to establish an effective and efficient system for handling grievances.
A Ministry of Defence (MoD) spokesperson said: "We have already accepted a number of recommendations to overhaul and improve the service complaints system, including encouraging personnel to come forward."
The MoD said the most serious allegations would now be handled by the Defence Authority, a central point in the department to allow complaints to be dealt with more quickly.