The Garratt couple, originally from Vancouver, run Peter’s Coffee House in Dandong, China’s main border town with North Korea. They have lived in China for about 30 years. 

Pictures of the cafe, which overlooks the Yalu river and the Friendship Bridge linking the two countries, show the “T” in its name is in the shape of a crucifix, with a backdrop resembling a stained-glass window.

Simeon Garratt used to live with his parents and brother in Dandong, but moved back to British Columbia a few years ago. His brother Peter, 21, still lives in Dandong. 

He said he had last spoken to his parents at 6.30pm on Monday as they were heading for dinner. Later that evening, his brother grew anxious when their 54-year-old father did not send photos from the dinner as promised. 

The parents’ phone was switched off later in the night. The parents were not at their apartment on Tuesday morning. 

The State Security Bureau in Dandong is carrying out the investigation into the allegations, Xinhua said. It did not say whether the Canadian couple were detained.  

Chinese law is vague in its definition of state secrets, describing them as any information that could “damage state security and interests in the areas of politics, economy, and national defence”.

The crime carries harsh penalties in China, ranging from 10 years in prison to the death penalty.

The Dandong region is a sensitive military area for China, and the border crossing is a key trade lifeline for nuclear-armed, diplomatically isolated North Korea.

It is also a focus for foreign Christian groups, including some from South Korea, with some working to assist North Koreans who secretly cross the border to escape from hardship and repression in their homeland.

Simeon Garratt described his parents as “openly Christian” and said they had been involved in sending goods such as oil and cooking supplies to impoverished North Korea to “help basically what they feel is a group of people that have been severely neglected”.

“I just think it’s crazy,” Garratt, who runs a software company in Vancouver, said of the investigation.

“It sounds like something somebody made up,” he said. “I really don’t know why. It’s just so absurd.”

On its website the coffee shop describes itself as “only metres from the border of North Korea”, and “the perfect stop off while en route to or returning from the Hermit Kingdom”.

Along with grilled sandwiches and quesadillas, the couple also organised trips to North Korea at their café, according to their website. They host a regular English corner for people wanting to practice their language skills and organise occasional concerts at the venue.

In an audio file posted on the website of the Terra Nova church in Surrey, British Columbia, Kevin Garratt tells the congregation: “We’re China based, we’re North Korea focused, but we’re Jesus centred.”

“God said, in a prayer meeting, go to Dandong and I’ll meet you there, and he said start a coffee house,” he said in a guest sermon dated November last year.

“We serve the best coffee on the border ... and we do some other things too."

“We’re trying to reach North Korea with God, with Jesus, and practical assistance.”

North Koreans regularly stay at a “training house” outside Dandong, he said, and “99 per cent of the people we meet go back to North Korea, because they have to preach the gospel in North Korea, because God has compelled them to go”.

The Canadian embassy in Beijing was aware of reports that two of its citizens had been detained, a spokesperson said. "Canadian consular officials stand ready to provide assistance, as required," he said. 

The Barratts returned to Dandong from a trip to Canada in June, their son said.

Calls to the café and a police station three buildings away on the same street went unanswered on Tuesday morning. 

This is the first time that Canadians have been detained on allegations of military espionage since the Cultural Revolution ended almost four decades ago, said Charles Burton, a professor of political science at Brock University and former Canadian diplomat in Beijing.

“If in the end it appears that they have been wrongfully arrested, a Canadian public outcry could put pressure on the Government to take action to express our national ire over this.”

The allegations come a week after Canada’s government said Chinese hackers had infiltrated the National Research Council of Canada and shut down the council’s network. The Chinese Foreign Ministry rejected the accusation, calling it "irresponsible”.

Last month, a British Columbia court denied bail to a Chinese resident in Canada charged with espionage.

Su Bin, a Chinese citizen living in Vancouver, has been accused by US authorities of running a spy network and attempting to obtain information on military aircraft from defence contractor Boeing. Su faces possible extradition to the US. His next court appearance is scheduled for August 27.

Canada’s Prime Minister Stephen Harper is expected to visit China in the autumn. 

Additional reporting by Teddy Ng