The UK Supreme Court has heard that the Welsh Government exceeded its devolved powers by proposin g a bill to reform local government bye-laws, reports the BBC.
The bill is the first piece of legislation proposed by the assembly since a referendum on devolution last year resulted in it being granted law-making powers over 20 areas.
Under the new arrangement the assembly does not need approval from the UK Government at Westminster to legislate in these areas.
The Local Government Byelaws bill was passed by the Welsh assembly in July, but was immediately blocked from becoming law by the Attorney General, Dominic Grieve QC, MP.
Counsel for the Attorney General, Jonathan Swift QC, told a panel of five Supreme Court justices that it was very clear that the bill exceeded the powers conferred on the assembly. He argued that it does so because it changes the powers of UK ministers.
Theo Huckle QC, defending the Welsh Government, pointed out that the powers that confirm certain council bye-laws are in practice obsolete, adding that the purpose of the bill was to streamline the process for instituting bye-laws, effectively allowing them to be made without the approval of the Welsh assembly in each case.
The case is continuing through the week, with a judgment expected on the matter before the end of the year. If the Court decides the bill is within the powers conferred on the assembly then it will immediately become law.
Mr Huckle believes that the Government's case against the legislation is based on the false misapprehension that UK ministers are asked to confirm Welsh bye-laws, something which he says never happens in practice.
"We are not aware of any instances when the Secretary of State has been requested to confirm Welsh local authority bye-laws," he said prior to the case commencing.
"It is therefore the view of the Welsh Government that the provisions of the bill to localise procedures for making, confirming and enforcing local authority bye-laws brings the bill within the legislative competence of the National Assembly for Wales," he added.