OTTAWA — Competition commissioner Matthew Boswell scored a huge win last week, when MPs on the House of Commons’s finance committee voted to make corporate mergers presumptively illegal if the deals would put more than 30 per cent of market share in one company’s hands.

Here’s what you need to know:

Where this came from: The fall economic statement included changes to the Competition Act to strengthen the commissioner’s powers. In March, Boswell wrote to parliamentarians to say those are “a generational upgrade in our competition law framework,” but he wanted more. 

Among his requests was a “structural presumption” that mergers leading to concentrations of market share over 30 per cent (or similar findings using a calculation of concentration called the Herfindahl-Hirschman Index) are anti-competitive. To get approval, merging parties would have to prove the opposite to the Competition Tribunal that hears the commissioner’s challenges.

NDP Leader Jagmeet Singh had a private member’s bill in the House of Commons that would do roughly the same thing.

What happened: About eight hours after the Commons finance committee got to work April 30, going over the bill implementing the fall economic statement, the NDP’s Don Davies proposed amendments to put a version of Boswell’s proposal into the law.

The committee approved them, following a further tweak from Liberal MP Ryan Turnbull to let the federal cabinet adjust the 30 per cent threshold as conditions warrant.

Why it matters: A reversed burden of proof would have totally changed the way Rogers’s takeover of Shaw played out, Boswell wrote in his March pitch, forcing them to propose “remedies” to the merger’s impact on competition earlier.

“Every negotiation before the tribunal [stage] happens in the shadow of what people think will happen at the tribunal,” Innovation Department competition expert Martin Simard told the MPs last week.

The move would also make Canada one of the world’s few countries to have such a provision: Germany has a threshold in law, while the U.S. has one as a guideline and in case law, but not codified.

Who likes it: “We’re very encouraged by the move to include structural presumptions against mergers in concentrated markets,” Keldon Bester of the Canadian Anti-Monopoly Project told The Logic.

Who doesn’t: In a paper for the C.D. Howe Institute, University of Toronto competition law professor Edward Iacobucci said it would be a mistake. (He was talking about Singh’s bill, but the principle is the same.) Just defining “market share” is fraught, he wrote—who decides where the limit of the market is?

Sometimes a merger creating a new big player will mean more competition, but such results can be hard to prove, he wrote.

The Canadian Bar Association’s competition-law section also criticized the reverse-onus provisions in Singh’s bill, on similar grounds.