Keen went public with complaints of political interference about phone calls and a letter she received from Lunn, threatening her dismissal.
Both the safety commission and Lunn's officer issued statements about Keen's firing. Assistant deputy industry minister Michael Binder has been named as her as interim replacement.
Lunn and Keen, who will remain on the board of the commission, were summoned to appear before the parliamentary committee. Keen says she will be there despite her dismissal.
The feud between Keen and the government started in December, when ongoing safety concerns prompted Keen's commission to shut down the Chalk River reactor, which is owned and operated by Atomic Energy of Canada Ltd. It produces more than one-half the world's supply of medical isotopes, which are used to diagnose cancer and other illnesses.
Shortly before Christmas, Parliament unanimously voted to overrule the nuclear safety regulator and order the reactor re-started.
In the statement issued by Lunn's office, the government said the extended shutdown of the reactor "was threatening to cause a national and international health crisis.
"The president was aware of the importance of maintaining Canada's and the world's supply of medical isotopes," the statement said. "However, given the growing crisis, she did not demonstrate the leadership expected of the president under the existing legislative provisions of the Nuclear Safety and Control Act to put the Commission in a position to address the situation in a timely fashion."
The Conservative government has blamed the commission's intransigence for creating the crisis. And Prime Minister Stephen Harper pointed a finger directly at Keen, a career bureaucrat whom he referred to as a Liberal-appointee.
In his letter to Keen (which is now public), Lunn said her handling of the Chalk River situation "cast doubt on whether you possess the fundamental good judgment required" by the head of the nuclear safety watchdog.
He indicated he was particularly irked Keen did not abide by a Dec. 10 ministerial "directive" to allow the reactor to start up again. And he said he was considering recommending to cabinet that her appointment be terminated.
Keen retorted in a blistering letter of her own, in which she accused Lunn of interfering with the independence of her quasi-judicial commission.
Opposition MPs have been demanding Harper fire Lunn, who hasn't spoken publicly on the issue for weeks, for his interference.
Liberals on the natural resources committee won support from the NDP and Bloc Quebecois members for a motion summoning Lunn and Keen to the special meeting.
Conservative members initially said they had no problem asking the minister to appear. But, after unsuccessfully insisting Lunn be given extra time to rebut anything Keen may say, only one of four Tories MPs supported the Liberal motion.
"The minister did a great job handling that issueÂ…. We think Canadians, when they hear the explanation will be very happy with it," said Conservative MP David Anderson.
Anderson suggested the Liberals are on the warpath against Lunn and should remember they supported the move to reopen the Chalk River facility.
Omar Alghabra, the Liberals' natural resources critic, said his party also wants to grill Lunn about when he first found out about the shut-down of the reactor. Some reports have suggested Lunn knew for weeks about the problem before alerting Health Minister Tony Clement in early December about the impact on isotope production.
"We had two issues here," Alghabra said.
"We had national public health and we had nuclear safety that were put at risk. So we need to understand, if the minister had knowledge, why didn't he act sooner."
"Second, we need to understand that we set boundaries and respect for our independent tribunals."