China has detained a Canadian man on suspicion of stealing and prying into state secrets, and released his wife, also a Canadian, on bail, after holding the couple without charge for months, China’s foreign ministry said on Thursday.

The decision to detain Kevin Garratt, who ran a Christian coffee shop with his wife, paves the way for his formal arrest and possible prosecution, in a case that has strained ties between Canada and China. Beijing is widening a crackdown on foreign Christian groups along its sensitive border with North Korea.

Foreign ministry spokesman Hong Lei said Garratt had been formally detained on suspicion of the theft, citing the National Security Agency of Dandong, a city in the north-eastern province of Liaoning, where the Garratts had lived for years.

“The relevant Chinese authorities are dealing with the case in accordance with the law, and maintaining the legal rights and interests of both people in accordance with the law,” Hong told a regular news briefing.

Julia Garratt was released but barred from leaving mainland China for one year, the family said in a statement. Kevin Garratt has been moved to a “more formal detention centre at an unknown location”, the family added.

No formal charges had been filed and no evidence of misconduct was given to the family or their lawyer, relatives said.

“The family continues to call upon the governments of Canada and the People’s Republic of China to resolve this matter involving diplomatic means with a sense of urgency,” the couple’s lawyer, James Zimmerman, said in a statement.

China’s state secrets law is notoriously broad, covering everything from industry data to the exact birth dates of state leaders. Information can also be labelled a state secret retroactively. In severe cases, the theft of state secrets is punishable with life in prison or the death penalty.

The Garratts were first detained last August for suspected theft of military and intelligence information and for threatening national security.

The Canadian prime minister, Stephen Harper, said in November he had raised concerns about the case in a meeting with the Chinese president, Xi Jinping.