Justice and policing should be devolved from Westminster to the Welsh Government in order to help prevent crime, Wales' top lawyer has said.
Counsel General for Wales Jeremy Miles AM told BBC Wales' Post Cyntaf that it made sense due to the close interaction between the law and devolved agencies.
He said it could cost £10m and take a dedace to fully devolve justice.
The Ministry of Justice (MoJ) said it would cost over £100m and "lead to duplication and inefficiencies".
Justice, including the courts, policing and the probation and prison services, is not currently devolved to Wales.
But Mr Miles, assembly member for Neath and the Welsh Government's chief legal adviser and representative in the courts, has argued that the way justice intersects with the most important devolved areas of policy means it should be.
"This isn't just a question of seeking powers for the sake of seeking powers - it is about having a coherent basis for devolution in Wales," he said.
No 'big bang'
In December last year, First Minister Carwyn Jones set up the Commission on Justice in Wales to review the operation of the justice system in Wales. It is due to report its findings next year.
Mr Miles said one of the questions the commission would explore is how long devolution of justice should take.
"The Welsh Government position has been that this might take 10 years to devolve fully - it is certainly not necessarily a 'big bang' approach," he explained.
A spokesperson for the MoJ added: "We work closely with the Welsh Government and devolved agencies and have the flexibility to adapt services specifically for Wales where required.
"This has ensured that the single legal jurisdiction continues to be the most effective and efficient way of administering justice across England and Wales."