A Quebec judge has agreed to hear the testimony of a prominent witness in a massive class-action lawsuit against Big Tobacco, a man the industry has labelled as biased and ill-informed.

Robert Proctor is a historian from California's Stanford University who has published extensively on the tobacco industry in books and academic papers.

He's also no stranger to tobacco litigation, having testified in some 30 trials.

He was called to testify on behalf of the plaintiffs behind a landmark $27 billion lawsuit in Quebec that pits an estimated 1.8 million Quebecers against three major tobacco manufacturers.

The defendants—Imperial Tobacco Canada Ltd.; Rothmans, Benson & Hedges; and JTI-Macdonald—have argued that the dangerous health effects of tobacco have been common knowledge for decades and there was no conspiracy to hide it.

Justice Brian Riordan decided late Monday that he wanted to hear from Proctor. He admitted as evidence part of his 100-plus page report, which critiques other reports done by three industry-paid historians on how much Quebecers knew about tobacco risks.

Lawyers for the tobacco firms spent the day attacking Proctor's credibility. They tried to convince the judge that the professor had an agenda beyond critiquing historians' reports.

The judge allowed about 30 pages into the record.

As for the rest of his report—which one tobacco lawyer described as 75 pages of anti-tobacco advocacy—Riordan said he would take it under advisement.

Proctor doesn't couch his words when describing what conclusions he draws about the tobacco industry from his research. He has described tobacco companies in writings as liars, cockroaches and cancer-mongers and he says cigarettes should be abolished.

Proctor did not hide his opinions Monday.

"I believe it is wrong for an industry to kill millions of people," said the author, researcher and self-described public health advocate, during a hearing to determine whether he should be granted expert status.

"I'm open to alternative views, but I'm not neutral about what your client has done to the lungs of the world."

Proctor, who has more than a quarter-century of experience, has testified in dozens of trials in the United States. He admits that he has earned more than $1 million for doing so. He has never once been disqualified from testifying.

"I'm fair, but I do think bad things have been done by the tobacco industry," Proctor said under questioning.

"I don't think the tobacco industry wanted to kill people. I think it was more negligence," he added. "I'm glad they are being brought to justice. That's a good thing."

His testimony is the latest in a case that is described as the biggest class-action lawsuit in Canadian history.

It's not common for experts to be excluded from testifying, one of the tobacco lawyers said. But he argued that the court had an obligation to prevent Proctor's testimony.

"The Supreme Court has said as recently as 2011 that the court has a gatekeeper function," said Doug Mitchell, a lawyer representing JTI-Macdonald.

He and other tobacco company lawyers argued that not only is Proctor biased, he knows nothing about Canada or Quebec, and he cites documents not in the record in his report.

Another lawyer, Simon Potter, representing Rothmans, Benson & Hedges, questioned whether Proctor's testimony was necessary.

"Is this person, this expert, going to help the court, as an expert should, with objective, helpful advice from a specialized field when the court is unable to decide without it?" Potter said.

"I think the answer is no."

But the lawyers representing two plaintiffs dismissed the arguments from Big Tobacco and said it wasn't Proctor's mandate to know Canadian tobacco history.

"Based on a lifetime's work, you have certain ideas, based on evidence," said plaintiff lawyer Bruce Johnston. "Surely that cannot disqualify you from working on a file because that would result in the most qualified people being excluded."

Plenty of witnesses have already appeared before the Quebec Superior Court since the trial began last March, including numerous former and current tobacco industry executives.

The case has already heard more than 80 days of testimony with thousands of pages of documents filed into evidence.

It has taken 13 years to reach the trial phase. It stems from two cases that were filed in 1998, certified and consolidated in 2005 by Quebec Superior Court, and there were motions, delays and appeals before it got underway in 2012.

Proctor's testimony on his report begins Tuesday morning.