OTTAWA (Reuters) - The Canadian Parliament was set to overrule the country’s nuclear watchdog on Wednesday and order a reactor that makes crucial radioisotopes for cancer tests be restarted immediately.

The Chalk River reactor -- which makes more than two-thirds of global supply of the medical isotopes -- was shut down in November, quickly triggering shortages.

The Conservative government, under heavy political pressure to solve the problem, is pushing through legislation that will allow the reactor to resume operations for 120 days.

This would involve using back-up safety systems, an action that the Canadian Nuclear Safety Commission says is unsafe.

The House of Commons approved the bill on Tuesday night and the Senate is expected to follow suit on Wednesday.

Once the reactor is restarted it will take three or four days before operator Atomic Energy of Canada Ltd can begin delivering isotopes. When injected into the body, the isotopes give off radiation that can be seen by a camera to diagnose cancer, heart disease and other medical conditions.

Government ministers insist there is no danger in restarting the reactor now.

“This was always an issue of public interest and it was the right thing to do. We’ve had absolute reassurance that we could resume production of medical isotopes, and 100 percent assurances of safety and that’s what our goal was all along,” Natural Resources Minister Gary Lunn told Reuters.

Prime Minister Stephen Harper strongly criticized nuclear commission boss Linda Keen on Tuesday, saying she was being unreasonable, but Keen did not change her opposition to the government’s plan.

“This provides significant risk not only to the reactor but to the employees and the communities that live around this reactor,” she told a special meeting of legislators late on Tuesday night.

At least one medical specialist hailed the emergency move, saying it would be welcomed by hospitals everywhere.

“This is a crucial thing, a great Christmas present for us, for sure,” said Dr Andrew Ross, a nuclear medicine specialist in the eastern Canadian city of Halifax.

“We have been living day to day,” he told the Canadian Broadcasting Corp. on Wednesday.

AECL, the government-owned nuclear technology company that operates the reactor, had earlier said it would not be back to full output until early to mid-January.

But on Tuesday it said “heroic efforts” by staff meant the facility could be restarted on December 20 without having to resort to the back-up safety procedure.

Chalk River produces medical isotopes for Canadian health care company MDS Inc and its MDS Nordion division, which is responsible for about 50 percent of world supply.