Seven unions challenged B.C.'s election spending laws in B.C. Supreme Court on Monday. (Mike Laanela/CBC)

The B.C. Supreme Court on Friday struck down a key part of the province's election gag law, which limits third parties from advertising three months before a scheduled election.

Bill 42 was designed to limit third-party political advertising in the 88 days before election day.

The law limits election advertising by any person or organization that is not a political candidate, riding organization or party to $3,000 in a single electoral district and $150,000 provincewide for the 88 days before an election.

The law was challenged by seven British Columbia labour unions that claimed it was unconstitutional.

Justice Frank Cole informed lawyers ahead of handing down his formal judgment that he sided with the unions, and will cut the restricted third-party advertising period from 88 days to 28 days. Cole's reasons for the ruling are expected to be released on Monday.

B.C.'s Attorney General, however, said he’s disappointed with Friday’s judgment.

"We could end up in a situation like the Americans where you have unregulated spending," Wally Oppal said. "I'll give you an example: the attorney general from Washington state spends two and a half million dollars to get elected."

Oppal plans to look over the ruling next week and then decide whether the province will appeal.

B.C.’s next provincial election is scheduled for May 12.

The groups behind the challenge were the Canadian Union of Public Employees (B.C. division), the B.C. Teachers' Federation, the Canadian Office and Professional Employees Union 378, the B.C. Nurses' Union, the B.C. Government and Service Employees' Union, the Federation of Post-Secondary Educators of B.C. and the Hospital Employees' Union.