Open this photo in gallery Canada’s ambassador to China, John McCallum. Sean Kilpatrick/THE CANADIAN PRESS

Chinese authorities are mistreating two Canadians detained last month for allegedly endangering national security, Canada’s ambassador to China told MPs in a closed-door session Friday.

Ambassador John McCallum told members of the House of Commons foreign relations committee that Michael Kovrig – a Canadian diplomat on leave – and businessman Michael Spavor are being kept in prison cells where the lights are on 24 hours a day, said sources at the hearing who were not allowed to publicly discuss what the envoy said. Authorities are also subjecting them to lengthy interrogations.

Mr. McCallum informed MPs at the in-camera session that China will only allow Canadian consular officials to visit the detained men once a month and only for half an hour. Authorities are monitoring their conversations and the Chinese guards forbid embassy officials from speaking to the Canadians in French in case they are passing on messages.

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At last week’s cabinet retreat in Sherbrooke, Mr. McCallum told reporters that the two Canadians face up to four hours of questioning each day and have no access to a lawyer, a situation that could last for up to six months under the Chinese legal system. Some observers believe the arrest of Mr. Kovrig breaches long-standing principles of diplomatic immunity, which are extended to former diplomats as well under the 1961 Vienna Convention on Diplomatic Relations, The Globe and Mail reported last Wednesday.

The two arrests are part of an escalating diplomatic crisis between the two countries after last month’s arrest by Canadian authorities of Huawei chief financial officer Meng Wanzhou in Vancouver at the behest of the U.S. government, which wants to extradite her so she can face fraud charges related to violating sanctions against Iran.

The arrest of Ms. Meng prompted sharp criticism of Canada from China and demands for the executive’s release; it came amid a continuing review by Canada into whether it should ban or limit the Chinese telecommunications giant from supplying 5G equipment to the Canadian wireless market like three of its allies in the Five Eyes intelligence-sharing network – the United States, Australia and New Zealand.

Security chiefs in the United States and former counterparts in Canada have warned the installation of Huawei 5G equipment could enable China to spy on other countries or modify or steal information, and even shut down systems. Canadian telecom giants BCE Inc. and Telus Corp. are Huawei customers and the two companies could face huge costs to swap out their Huawei equipment should the government decide to ban the company from Canada’s 5G networks. Telus defended its decision to use Huawei equipment in a memo to employees last week.

Tensions with China soared last week when China’s ambassador to Canada Lu Shaye publicly stated that Canada would suffer repercussions if Ottawa were to ban Huawei. He also warned Foreign Affairs Minister Chrystia Freeland against rallying international support for the two detained Canadians and seeking clemency for another Canadian whose 15-year sentence by a Chinese court for a drug smuggling conviction was changed to the death penalty by an appeals court. Prime Minister Justin Trudeau called the application of the death penalty “arbitrary” and said last week after China’s actions: “We are in a situation where the application of the rule of law is not at the forefront.”

Although the growing rift between Canada and China has dominated headlines in recent days, Mr. Trudeau did not mention it during his kick-off speech on Sunday at the start of two days of Liberal caucus meetings in Ottawa. He made only an oblique and passing reference to China, saying “the world’s two largest economies” – the United States and China – “are at odds.” Three people who attended the meeting, including Finance Minister Bill Morneau, said the topic of China did not come up during their in-camera meeting, either. “The discussion today was with our caucus around a number of things around our platform objectives, our goals in the political process,” Mr. Morneau told The Globe. When asked if China was discussed, he replied: “Not when I was in the room.”

Instead, the Prime Minister delivered an attack on the opposition Conservative Party, making clear he plans to fight this fall’s election by sticking to an appeal to the pocketbooks of middle-class voters – and by evoking the name of his prime ministerial predecessor Stephen Harper as much as the current opposition leader Andrew Scheer.

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NDP MP Nathan Cullen criticized Mr. Trudeau’s approach to China as “dangerously naive” and said the message he should deliver to his MPs was “that he has a plan, not only for this Canadian on death row, but that he has a plan to deal intelligently and maturely with [China]. I’m just a bit surprised, but I assumed the Liberals had some better and deeper insights into the way Chinese politics and business manifests and that they wouldn’t allow themselves to be trapped this way and literally risked the lives of Canadians because of it.”