A criminal lawyer says there needs to be a stronger system of tracking police officers who resign during misconduct investigations.
In some cases, officers who resign from one force can get a job in a different force before investigations conclude.
In police forces in Wales between 2009 and 2011, 25 officers resigned while under investigation.
Campaigning lawyer Stuart Hutton believes a database should be created to monitor officers' track records.
The Home Office said procedures had been strengthened, but responsibility lay with individual forces.
There is concern that some of the officers who resign during an investigation will go on to work for a different force without the investigation being completed.
Some experts believe the loophole has a negative effect on forces' integrity.
Figures show that between 2009 and 2011, 18 South Wales Police officers resigned during investigation.
North Wales Police had four officers resign while under investigation, and there were three in Gwent.
While no Dyfed-Powys Police officers resigned during an investigation, two were dismissed and eight received a written warning.
Mr Hutton, a campaigner on miscarriages of justice and human rights issues, said: "Justice hasn't been seen to be done in the sense that the officers have left.
"The matter then becomes buried and perhaps is in danger of becoming lost and the consequences of that is obvious.
"No record is kept, which is not helpful for successive employers or successive police forces who may employ that person who's being investigated."
He added: "If there is an investigation, I think the police force should transparently parade in a proper manner the people that they are investigating and the outcome of that investigation.
"If a complaint has not been upheld because somebody has left the police force before that opportunity has arisen, then the police should have an intelligent database which may not be public."
Mr Hutton said the rules need to be tightened up and suggested a possible merging of the Inspectorate of Constabulary and Police Complaints Commission.
"A regulatory authority would provide a cast iron decision," he added.
"We need regulations so we know exactly what the rules are, serving police officers know what the rules are, and it transcends the internal views that different police forces have in terms of interpreting their obligations to the community.
"We spend a lot of money on our police forces and we do need to restore public confidence."
South Wales Police said the force had "a dedicated professional standards department who ensure we are progressive and robust in responding to allegations, conducting complex investigations into officers whose behaviour or lifestyle presents a cause for concern".
North Wales Police said every case "would have been considered carefully" and there were "few occasions" where accepting resignations was appropriate.
The Gwent force said any reports of improper conduct by officers or staff would be fully investigated by the professional standards department or, where appropriate, be referred to the Independent Police Complaints Commission.
Dyfed-Powys Police's website says officers should behave "appropriately at all times", and must "act in a manner that does not discredit or undermine public confidence in the police service".
A Home Office spokesperson said: "The Home Office and police service have strengthened the mechanisms around dealing with police misconduct and vetting of police officers.
"Responsibility for day-to-day policy, operational decisions and individual cases lies with police forces."
But Dr Tim Brain, an honorary senior research fellow at Cardiff University and a former chief constable of Gloucestershire, said that while a resignation would mean the whole process including a disciplinary hearing would not be able to be completed, an investigation could be, and a police force could issue an apology.