The federal Liberal and Green parties called for the resignation of Natural Resources Minister Gary Lunn Wednesday in the wake of what they call his interference with the country's nuclear watchdog.

"We believe the minister should be fired," Liberal Leader Stéphane Dion said during a press conference.

Lunn is engaged in a public spat with Linda Keen, the head of the Canadian Nuclear Safety Commission (CNSC) over the November shutdown of a 50-year-old reactor that generates two-thirds of the radioisotopes used around the world for medical imaging.

The closure of the Chalk River, Ont., reactor resulted in a global shortage of the nuclear material. Lunn directed the CNSC to reopen the site on Dec. 10, but the agency refused, insisting a backup safety system needed to be installed to prevent the risk of a meltdown during an earthquake or other disaster.

A Dec. 11 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.

In a letter dated Dec. 27, leaked to media and now posted on the CNSC website, Lunn threatened to fire Keen and wrote that Keen's response to the situation "cast doubt" on whether she possessed the "fundamental good judgment" to run the commission.

Keen responded via a letter, also available on the commission's website, accusing Lunn of improper interference.

The Liberal leader sided with Keen in the disagreement, saying Lunn tried to "scapegoat" the commission head and that a parliamentary committee should be tasked to investigate.

Dion also said Prime Minister Stephen Harper should apologize for putting into question the impartiality of an arm's-length tribunal and that the Conservative government needs to learn "when partisanship must stop."

The situation could create a "very bad relationship with all the administrative tribunals in Canada," Dion said. "This is damaging all democracy, big time."

Greens say Lunn should resign

In a release calling for Lunn's resignation, Green party Leader Elizabeth May said Lunn's threats to Keen "cross the line of appropriate political deference to an independent regulator" and "weaken the credibility of nuclear safety in Canada."

"It is unprecedented in Canadian political life for politicians to demand certain decisions from regulators," the release said.

In addition to Lunn's resignation, the party also called for a full inquiry into the nuclear industry, including its safety record and value for money.