China's security services have been sending undercover agents into Canada on tourist visas to strong-arm expatriates to return home, including some suspected of corruption and other criminal activities.

The secret Chinese visits have raised concern among lawyers and prompted investigations by the Canadian Security Intelligence Service and the RCMP, even as the Trudeau government begins negotiations for an extradition treaty with China.

According to an insider briefed on China's secret-agent operation, the Chinese moved to tactics that include threats and intimidation because they were "ticked off" at Canada for "not being willing to send people back the instant they asked" and for dragging its feet on an extradition treaty.

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"Nobody has been caught and nobody has been taken to court," the insider said.

The revelation comes as Chinese Premier Li Keqiang arrives in Ottawa on Wednesday for talks with Prime Minister Justin Trudeau. The two leaders are expected to discuss the extradition treaty between China and Canada.

Formal talks on the extradition treaty were approved in Beijing on Sept. 12 between a top Communist Party official and Daniel Jean, national security adviser to Mr. Trudeau. A day later, a Chinese court ordered the deportation to Canada of jailed Canadian missionary Kevin Garratt.

China experts and opposition critics say Mr. Garratt's release appears to be a quid pro quo for Canada beginning talks on an extradition treaty.

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The covert Chinese operation has been going on for years in Canada, according to the insider briefed on the situation. It is part of a global effort by China to repatriate fugitives and recover money stolen by Communist party officials or employees of state-owned enterprises.

U.S. diplomats warned China in 2015 to stop using its security agents on American soil to pressure Chinese citizens to return home to face its court system.

While Mr. Trudeau was in China for an official visit earlier this month, The Globe asked him about the tactics Chinese agents are alleged to have used. In response, Mr. Trudeau said such issues are "exactly why" the two sides established a high-level dialogue on security and rule of law with China, "which will allow Canadian officials and Chinese officials to discuss specific cases, to discuss the principles and concerns that both sides have."

Toronto refugee and immigration lawyer Lorne Waldman told The Globe and Mail some of his Chinese clients in Canada have received cellphone messages from Chinese security officials threatening them and their families if they do not return home. One client suspected that people watching him "might be from the Chinese government," he said.

Mr. Waldman said he has worked with at least six people sought by China as fugitives. In the past year, two of them and the family of a third have been approached by CSIS agents who "asked them whether or not they had suffered harassment at the hands of those Chinese officials in Canada … in relation to their return to China, or made any threats directed toward them."

The most recent such meeting took place this summer when "the officers told my clients that CSIS was investigating whether or not this type of harassment is occurring," he said.

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In the House of Commons on Tuesday, the Conservatives and New Democrats questioned the wisdom of repatriating Chinese citizens back to a country with a death penalty and a justice system that includes a reliance of evidence obtained through coercion and where the courts fall under the jurisdiction of the Communist Party.

Interim Conservative leader Rona Ambrose noted that in August Immigration Minister John McCallum had rejected a formal extradition treaty as long as China has a death penalty.

"Today the Prime Minister is at the table hammering out an agreement. I follow the news, Mr. Speaker, and I don't see any headlines that China has abolished the death penalty, so what has changed in China month?" she said.

NDP Foreign Affairs critic Hélène Laverdière said: "Is this what the Prime Minister calls standing up for human rights?"

Mr. Trudeau told reporters at the United Nations that Canada will put in safeguards in any extradition treaty but did not provide details.

"Extradition is certainly one of the things the Chinese have indicated they want to talk about," he said. "But any discussions around extradition, for example, will be very much in line with Canadian principles and Canadian values and Canadian expectations that are very high."

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Canada's concerns with Chinese agents sneaking into the country are long-standing. In 2000, three secret police investigators applied for visas to come to Canada as workers for China National Pulp & Paper Corp., saying they wanted to discuss "Chinese users' requirements for Canadian pulp and paper."

It was only later that Canada learned they came to pressure Chinese businessman Lai Changxing to return to China, where he was wanted on smuggling and bribery charges. The three police were accompanied by Mr. Lai's brother.

The incident prompted Canada to file a diplomatic protest, said David Matas, a lawyer who represented Mr. Lai. Once called "China's most-wanted fugitive," Mr. Lai was deported in 2011 and sentenced to life in Chinese prison the following year.

Lawyer Clive Ansley, who has served as an expert witness on China's legal system in deportation hearings, called an extradition treaty with Beijing "an atrocious idea."

"Torture is routine in the Chinese criminal justice system," he said. "The idea of Canada sitting down at the table and smiling nicely and entering into new extradition treaties to send people back – to me, it's just beyond the pale."