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A bill to reform local government bye-laws exceeded the Welsh assembly's powers, the Supreme Court has heard.

Lawyers for the Welsh and UK governments clashed in court over the first piece of legislation passed since last year's referendum.

The Local Government Byelaws Bill was passed by the assembly in July this year.

But it was blocked from becoming law by the attorney general and referred to the court.

QC for the Attorney General, Jonathan Swift, told the court it was "plain as a pikestaff" that the bill exceeded the assembly's powers by changing the powers of UK ministers.

Under the law that governs the assembly's operation, the assembly cannot do this unless it is "coincidental on or incidental to" other parts of legislation.

But Welsh government counsel general Theo Huckle QC defended the bill's lawfulness, pointing out that powers to confirm certain council bye-laws which were changed by the bill were "in practical terms, obsolete".

He said the bill's main purpose was to streamline the way bye-laws are made by removing the need for Welsh ministers to confirm them.

Under the devolution settlement of 1999, UK ministers retained what is called a "concurrent" function in some specific circumstances, and it also removes these for the same reason.

Mr Huckle said the devolution journey had progressed significantly since 1999, and that it was within the assembly's powers to take the step under the full law-making powers which were introduced following the 2011 referendum.

He said the bill's reference to the court by the attorney general was based on a "misapprehension".

A panel of five Supreme Court judges, headed by the court's president, Lord Neuberger, heard several hours of technical legal argument from both sides.

Mr Huckle will continue giving evidence on Wednesday and the court will also hear from the assembly commission and the attorney general of Northern Ireland later in the week, with a judgement expected before Christmas.

If the court finds that the bill is lawful, then it will progress to Royal Assent and become law.

If the UK government succeeds in getting a declaration that parts of it are unlawful, the Welsh government will face the prospect of another vote in the assembly to amend it before it can become law.

Meanwhile, a committee in the assembly has changed its procedures in an attempt to avoid a repeat of the case.

In future members of the Constitutional and Legislative Affairs Committee will ask ministers whether any issues of competence have been raised about proposed legislation.