A group of B.C. unions has followed through on its pledge to go to court to fight a provincial government law that will limit third party spending during election campaigns.

Seven unions have filed a Charter challenge of Bill 42, claiming it violates the right of freedom of expression by restricting political debate before and during an election campaign.

They also claim the law violates an individual's right to vote by preventing people from bringing their concerns to the attention of politicians.

The law will impose spending limits on third-party advertising during the 28-day campaign and for 60 days beforehand.

B.C. Federation of Labour President Jim Sinclair says democratic governments should be protecting the rights of people, not trampling on them, adding Bill 42 is Premier Gordon Campbell's attempt to silence critics and control the airwaves ahead of the next election.

Attorney General Wally Oppal has defended the law, saying the Supreme Court of Canada has sanctioned election spending limits as a means of fairness.