Prime Minister Stephen Harper ignored demands that he owed a Liberal MP an apology on Thursday, one day after suggesting the Grits are opposed to anti-terror measures in order to protect one of their parliamentarian's relatives.

Liberal MP Navdeep Bains demanded an apology from the prime minister during question period.

"Yesterday, the prime minister in the House of Commons attacked my integrity and the integrity of my family. Now that he's had some time to think, will the prime minister simply retract his remarks?" Bains asked.

An unrepentant Harper sidestepped the question, instead saying he recognized the Air India bombing was an important matter.

"While I don't accept the premise of the honourable member's question, I will say this: that I will take, and I think this government will undertake, any action necessary to ensure that we put in place the measures to allow the police to do their investigation and to ensure that these things never occur again."

Bains rose in the House once again to repeat his request that the prime minister retract his comments. Harper effectively challenged him to point to any error in the newspaper report.

"If the honourable member denies any particular element in that Vancouver Sun story, I'd be more than happy to accept his word on the matter," Harper said.

The political firestorm erupted one day earlier when Harper attempted to read from a newspaper report saying that Mississauga-Brampton South MP Navdeep Bains' father-in-law is on a list of potential witnesses that the RCMP wants to compel to testify about the 1985 bombing.

Harper had been responding to unrelated criticism about the way his government chooses judges.

"Obviously, the Liberal Party opposes the change we have made ..." he said.

"I am not surprised, given what I am reading in the Vancouver Sun today, when I realize this is how the Liberal Party makes decisions."

He then began reading: "The Vancouver Sun has learned that the father-in-law of the Member of Parliament for Mississauga-Brampton ..."

Liberals shouted down Harper with cries of "Shame! Shame!" and said he was tarnishing the reputation of Bains and members of his family.

After the furor in the Commons, Harper's press secretary, Dimitri Soudas, told reporters the prime minister had not been suggesting the Liberals' decision to oppose the measures was linked to the MP's father-in-law.

But the Liberals weren't buying it.

"Yesterday, in this house the prime minister's behaviour disgraced this institution. It was a disgrace that the prime minister called into question the integrity of a member of parliament and his family, without a sliver of proof," Liberal Deputy Leader Michael Ignatieff said Thursday.

"And it was a disgrace that he tried to stigmatize a possible witness of an investigative hearing. ... This is a prime minister who apparently will say anything to get elected and will possibly do anything to hold power. Will the prime minister apologize to the House and to the Canadian people?"

Harper once again denied "the premise" of the question.

"I repeat: this government, this prime minister, will take any course of action necessary if we can get the Liberal Party to change their wrongheaded position and to make sure the police investigation of the Air India bombing can continue," Harper said.

Bains has said he didn't know his father-in-law Darshan Singh Saini was on a list of potential witnesses.

Meanwhile, a group representing families of victims of the Air India bombing is urging federal politicians to support renewal of Canada's Anti-Terrorism Act. The Air India Victims' Families Association spokesman Bal Gupta says fighting over the provisions is an insult to the people killed in the 1985 Air India bombing.

Harper has accused Liberal Leader Stephane Dion with hampering a police investigation into the bombing by rejecting provisions of the Anti-Terrorism Act.

Dion says his party will oppose extending the provisions when they expire on March 1.

The provisions would allow preventive arrests and special investigative hearings. The Mounties had hoped to use the investigative-hearing provision of the act to compel testimony.

The Conservatives have been trying to exploit a division in the Liberal Party over the matter. But the issue appeared to help galvanize the Liberals to act as a united front on the anti-terror measures.

Dion is whipping the caucus to oppose the provisions, he said on CTV Newsnet's Mike Duffy Live.

"It's a decision of the caucus, and we expect a lot of discipline from everyone. I am a very collegial leader, I consult a lot," he said Thursday.

"And when we come to a decision, when we think it's the best one for the Canadian people, then we act collectively," he said.

When asked whether Liberals who oppose the decision will be kicked out of the caucus, Dion declined comment on what he termed a "hypothetical situation."

With files from The Canadian Press