Prime Minister Justin Trudeau rejected repeated opposition demands on Monday to reveal the names of other cabinet ministers who used the same loophole as Finance Minister Bill Morneau to avoid divesting personal investments or putting them in a blind trust.

The Office of Conflict of Interest and Ethics Commissioner Mary Dawson told The Globe and Mail that a handful of cabinet ministers have managed to retain control of assets they would be required to divest if this wealth was not held indirectly through a holding company or similar mechanism.

Ms. Dawson's office, citing confidentiality rules, declined to identify the ministers but said "fewer than five cabinet ministers currently hold controlled assets indirectly."

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The issue, first reported by The Globe on Monday, dominated the Commons Question Period.

Read also: Ethics watchdog says other Trudeau ministers using same loophole as Morneau

Opposition Leader Andrew Scheer pressed Mr. Trudeau to name the other cabinet ministers "using the exact same loophole. It is a very simple question. Who are they?"

Mr. Trudeau would not reveal any names despite numerous requests from opposition MPs, calling the line of inquiry nothing but "petty personal attacks."

"The Finance Minister, all ministers in this House … follow the advice and recommendations of the Conflict of Interest Commissioner," the Prime Minister told the Commons. "The conflict of interest and ethics commissioner is there to ensure that, above all of these petty personal attacks, Canadians can be confident that people follow the rules."

The refusal of both Ms. Dawson and the Prime Minister to reveal names make it difficult to know for certain who is using the loophole.

"This is some sort of shell game going on where you have to guess at who might or might not be in a conflict of interest," NDP ethics critic Nathan Cullen told reporters.

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Conflict-of-interest filings for Justice Minister Judy Wilson-Rayboud and International Trade Minister François-Philippe Champagne show they have interests in private corporations but their offices say they do not directly or indirectly hold publicly traded shares. Veterans Affairs Minister Seamus O'Regan has publicly traded securities in a retirement account and his filing with the ethics commissioner shows they have not been divested or put in a blind trust.

Conflict-of-interest law in Canada requires cabinet ministers to divest assets such as publicly traded shares by selling them in an arm's-length transaction or putting them in a blind trust until they leave office. The exception, according to the ethics commissioner's office, is if these shares or similar assets are held indirectly through a holding company or similar mechanism.

Ms. Dawson had asked the Harper government in 2013 to change the law to eliminate this loophole but the Conservatives failed to make her suggested change. The bipartisan Commons ethics committee passed a motion on Monday to study reforms to the Conflict of Interest Act.

Finance Minister Bill Morneau used that loophole to place one million shares of Morneau Shepell in a numbered Alberta company after his election to parliament in 2015. The minister faced mounting criticism after The Globe revealed Oct. 16 that he had not placed his holdings in a blind trust. Mr. Morneau has since reversed course and announced he would put all his assets in a blind trust, sell off his entire portfolio of Morneau Shepell shares and donate to charity the $5.3-million in profit these stocks had earned since he became Finance Minister.

Ms. Dawson is also reviewing Mr. Morneau's involvement in drafting Bill C-27, which proposes changes to private pensions that could benefit major players in the human-resources and pension-management industry including Morneau Shepell.

Mr. Morneau's shares in Morneau Shepell rose by nearly $1-million in value in the first few days after he introduced legislation to rewrite federal pension law that he had championed and advocated for while still in the private sector.

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When he was still executive chair of Morneau Shepell, the company sponsored a Public Policy Forum summit on pension reform in October, 2014, that talked up the measures he included in Bill C-27.

Back then, one year before winning office, Mr. Morneau delivered a keynote speech lauding the benefits of moving away from defined-benefit pension plans to target-benefit plans, which lower the monetary liability for employers by shifting risk to employees .

"In a world where only the broader public sector and a few large corporations have [defined-benefit] plans, and citizens are feeling very stressed about their financial situation, it will be difficult for government to deal with these plans in their current design," he said. "Reducing the costs of these plans, by considering ideas like target-benefit plans or shared-risk pensions, which limits the guarantee and the reduce the cost, may be the best way to sustain the very positive security aspect of these programs."

C-27 would enable federally regulated businesses to create target-benefit pension plans and the opposition parties argue Morneau Shepell stands to benefit because existing clients might want to switch to target-benefit plans from defined-benefit plans. Also, critics say, the new legislation would require actuarial valuations every year, which could also mean more work for companies such as Morneau Shepell.

The proposed law was tabled on Oct. 19, 2016. That day, Morneau Shepell shares opened trading at $19.48. On Monday, the shares closed at $21.44, which means Mr. Morneau's one million shares are worth in excess of $1.9-million more than the day C-27 was tabled.

The same day its former executive chairman tabled Bill C-27 in 2016, Morneau Shepell released a statement praising the bill as a "positive step that could boost the development of these types of plans across the country."

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Last Friday, Morneau Shepell released another statement saying it did not lobby Ottawa on Bill C-27 and claimed the impact of the legislation is "not expected to have a material impact on our company."