The Conservatives have opened up a new line of attack against Bill Morneau, accusing the embattled finance minister of selling off stock the week before a policy announcement drove their value down by half a million dollars.

Inside the House of Commons today, Tory MPs peppered Morneau with questions about the timing of the sale of $10 million worth of shares in his family-founded human resources firm, Morneau Shepell.

Conservative finance critic Pierre Poilievre says a motion introduced by Morneau in December 2015 to raise income taxes on the highest earners caused the entire stock market to drop — including Morneau Shepell's share price.

Poilievre asks if Morneau used insider knowledge2:45

Poilievre asked Morneau repeatedly throughout question period if it was merely a coincidence that the 680,000 shares were sold off a week before the announcement.

Morneau largely avoided the queries, insisting he had absolutely nothing to hide.

A spokesman for Morneau later called Poilievre's insinuations "utterly false" and "absurd" — and challenged the Tories to make the accusations outside the House of Commons, where members are not protected by parliamentary privilege.

Poilievre on his latest questions to Morneau1:54

The NDP's ethics critic, Nathan Cullen, has written a letter to Ethics Commissioner Mary Dawson, requesting she investigate this matter. 

"If the finance minister used his inside knowledge to sell his shares at an advantageous time to financially profit, it would be in direct violation of the rules that prevent someone from profiting directly from their work in government," Cullen wrote in a letter that was sent Monday afternoon.

"I respectfully ask that you look into this matter as urgently as possible," the letter says.

The Ethics Commissioner is already investigating Morneau about whether he was in a conflict of interest when he sponsored a bill related to pension reform.

Morneau still owned shares in his family business, Morneau Shepell, a benefits and pension services firm, at the time the bill was introduced.

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with files from CBC's Katie Simpson