MONTREAL/OTTAWA -The union representing workers at Canadian National Railway said on Friday they would strike next week, in a fresh threat to the economy after vowing to challenge a government decision to end an unprecedented rail stoppage.

The Teamsters union filed notice that conductors, locomotive engineers and other workers at Montreal-based CN would strike on Monday, just days after returning to work on Friday.

The announcement is the latest twist in a labor dispute at Canada's top two railroads, which locked out more than 9,000 unionized workers on Thursday, triggering a simultaneous rail stoppage that business groups said could inflict hundreds of millions of dollars in economic damage.

Labour Minister Steven MacKinnon, citing the risk to the economy, asked the country's industrial relations board on Thursday to order an end to the stoppage and also impose binding arbitration on the union as well as CN and rival Canadian Pacific Kansas City.

Canada, the world's second-largest country by area, relies heavily on trains to transport a wide range of commodities and goods. Business and agricultural groups had pressed Ottawa to act quickly.

Francois Laporte, president of Teamsters Canada, said MacKinnon's announcement was unacceptable.

"The best way to have a contract is at the bargaining table. We don't believe a third party (should) decide what are going to be our working conditions," he told reporters at a picket outside CPKC's Calgary headquarters.

He said that in case of a back-to-work order, "our people will still be on strike. We will still be on the streets, so operations will not resume. It's not going to be business as usual for both companies".

In their strike notice, the Teamsters said they were still prepared to negotiate with CN over the weekend and did not see any issues as insurmountable. The union and companies disagree on issues such as safety and scheduling.

A Canadian government official had no immediate comment.

TRAINS RUNNING AGAIN

A CN spokesperson said trains were starting to run and the company's plan to resume operations was under way.

"We are focused on getting back to work," said Jonathan Abecassis, CN's spokesperson. "The Teamsters are focused on getting back to the picket line."

A lockout at CPKC has yet to be officially lifted after Ottawa moved on Thursday to end the stoppage, which for the first time involved both railways simultaneously.

MacKinnon said on Thursday he expected the industrial relations board to issue a ruling in the near future. Union and railroad officials met with the CIRB, an independent body, on Friday morning, the Teamsters said on social media site X.

Teamsters spokesperson Chris Monette said by phone earlier in the day that the union would challenge the constitutionality of the minister's referral, without giving details.

MacKinnon had expressed confidence that his referral would survive a court challenge, given the broad power he has under the country's labor code.

"I can't imagine it ... would more practically or appropriately be used than in the case of two national railways ceasing their operations. So we're very, very confident about the path that we've selected here," he said.

Monette said CN workers would return to work on Friday, although the union had not received any back-to-work protocol from the railway.

"The return to work will be chaotic this morning," he said.

Monette said members from CPKC would not be back at work as the railway had not yet ended its lockout.

CPKC said late on Thursday that it was preparing to restart operations in Canada and that further details on timing would be provided once it received the CIRB's order.

  - Reuters