Natural Resources Minister Gary Lunn should be "commended" for his decision to re-open a nuclear reactor against the advice of the country's nuclear watchdog in order to relieve a worldwide shortage of medical isotopes, the prime minister said Thursday.

"I think the minister of natural resources, in fact all the ministers of the government who worked on this file, are to be commended for acting, frankly, beyond the normal call of duty to ensure that the Canadian medical system was not needlessly endangered by decisions made by the president of the nuclear commission," Stephen Harper said during a New Brunswick press conference.

His statement of support comes a day after the Liberal and Green parties called for Lunn's dismissal or resignation for what they described as improper interference with the Canadian Nuclear Safety Commission (CNSC).

In December, Lunn oversaw the restarting of the Chalk River, Ont., nuclear reactor, which is responsible for generating two-thirds of the radioisotopes used around the world for medical imaging.

Harper 'troubled' by CNSC refusal

The 50-year-old reactor was shutdown in November, resulting in a global shortage of the nuclear material. Lunn directed the CNSC to reopen the site on Dec. 10, but the watchdog refused, insisting a backup safety system needed to be installed to prevent the risk of a meltdown during an earthquake or other disaster.

The following day, an emergency measure passed through the House of Commons overturned the watchdog's decision, and the reactor was restarted for a 120-day run on Dec. 16.

Harper said he is "very troubled" by commission head Linda Keen's refusal to restart the reactor.

"This is an incident that should never have happened and cannot be contemplated in the future," Harper said. He said the move to reopen the facility, which passed unanimously in both houses of Parliament in less than 48 hours, was necessary.

"To label that kind of action illicit troubles me greatly," the prime minister said, referring to the opposition's calls to dismiss Lunn over the move.

On Wednesday, Liberal leader Stéphane Dion said he believed Lunn should be fired because the move threatened the impartiality of arm's-length tribunals. The same day, Green Party Leader Elizabeth May issued a news release calling for Lunn's resignation because he "cross[ed] the line of appropriate political deference to an independent regulator."

Lunn has been involved in a public spat with Keen and threatened to fire her in a letter that became public Tuesday. Keen in turn has accused Lunn of improper interference.