Overriding the safety concerns of its regulator, the Canadian government is rushing through legislation to reopen a nuclear reactor that produces most of the world’s supply of a medically important diagnostic isotope.

Hospitals worldwide have canceled or delayed thousands of nuclear medicine tests used to diagnose a wide array of illnesses because of the prolonged shutdown of the reactor in Chalk River, Ontario, near Ottawa.

The legislation, which was pushed through the House of Commons late Tuesday night, will suspend the Canadian Nuclear Safety Commission’s oversight of the reactor for 120 days and allow Atomic Energy to restart it immediately. The bill was approved by Canada’s unelected Senate on Wednesday night.

In a letter submitted to the government this week, Linda J. Keen, the president of the safety commission, said the Chalk River reactor, which is known as the National Research Universal, or N.R.U., had an unusual design that required its cooling-water pumps to operate at all times. The emergency backup for those pumps, she added, “is essential for the safe operation of the N.R.U. reactor.”

The agency had planned to replace the aging reactor and prevent disruptions to the medical isotope market by building two new reactors.