Image caption Unions have welcomed the latest court ruling

A judge in the US state of Wisconsin has thrown out a controversial law that strips most public employees of their collective bargaining rights.

The law, championed by Republican Governor Scott Walker, has been in effect for more than a year.

A Dane County circuit judge ruled that key parts of the law violated the constitution and were null and void.

Unions welcomed the ruling but Mr Walker's administration immediately vowed to appeal.

In a statement, Governor Walker described the judge as a "liberal activist" who "wants to go backwards and take away the lawmaking responsibilities of the legislature and the governor".

"We are confident that the state will ultimately prevail," the statement said.

It is unclear whether implementation of the legislation will be suspended pending the appeals process.

'Separate classes'

Correspondents say the latest ruling throws into question changes that have been made in pay, benefits and other work rules across the state for city, county and school district workers.

The lawsuit had been brought by two Wisconsin unions representing teachers and public workers.

In his ruling, Judge Juan Colas said sections of the law infringed "the rights of those employees who choose union membership", as well as the rights of free speech and association "guaranteed by both the Wisconsin and United States constitutions".

The judge said the law also violated the "equal protection clause" of the US constitution by creating separate classes of workers who were treated differently and unequally.

In June last year Wisconsin Supreme Court ruled that the law was constitutional.

Correspondents say it now looks set to return to the Supreme Court.

In June, Mr Walker survived an attempt to oust him from office in a recall vote - only the third such vote in US history.