Academics hired by the tobacco industry to paint a historical portrait of how much the public knew about the harmful effects of tobacco use left out an important element, according to a witness at a class-action trial: internal documents from the companies themselves.

Robert Proctor is testifying for the plaintiffs before the Quebec Superior Court at a landmark $27 billion lawsuit that pits an estimated 1.8 million Quebecers against the country's three major tobacco manufacturers — Imperial Tobacco Canada Ltd.; Rothmans, Benson & Hedges; and JTI-Macdonald.

Proctor, a historian from California's Stanford University, is a self-described cigarette historian and public-health advocate. He has published extensively on the history of smoking, tobacco and health.

He says that historians hired by Canada's big three tobacco companies to file reports for the class-action suit produced documents laden with flaws and omissions that Proctor says are significant.

Proctor says that by ignoring the industry data, they paint a false picture that people were aware 50 years ago of the effects of smoking when, in fact, the internal documents say the opposite.

He says that led to incomplete conclusions.

"All three [historians] fail to consult the tobacco industry's internal documents, which reveal a decades-long conspiracy to downplay the hazards of smoking," Proctor wrote.

"All three ignore the tobacco industry's denialist campaign, which in the global aggregate must figure as one of the deadliest conspiracies in the history of human civilization."

Proctor's official testimony began Tuesday with frequent objections from lawyers representing the tobacco industry.

On Monday, they'd fought against Proctor receiving expert status, labelling him as biased and unqualified to discuss the tobacco industry in Canada and Quebec.

Proctor's base of knowledge is largely U.S.-based, but he says the trends in both countries are similar. Justice Brian Riordan agreed late Monday to allow Proctor to testify.

The U.S. historian's testimony is expected to continue at least until the end of this week.

Another star witness, Jeffrey Wigand, a former tobacco executive-turned-whistleblower and was portrayed in "The Insider," a major motion picture in 1999, by actor Russell Crowe.

Wigand is scheduled to appear in December.

Numerous witnesses have already appeared before the Quebec Superior Court since the trial began last March, including former and current tobacco industry executives. The case has 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.

The trial is expected to last about two years.