China's decision to investigate two Canadians for suspected spying highlights a sharp and unexpected deterioration in bilateral ties just months ahead of a trip by Canada's Prime Minister Stephen Harper to Beijing.

The detention of the couple, longtime residents in China who ran a coffee shop on the border with North Korea, could also make life tougher for the small group of Canadian government ministers who favor closer economic links with Beijing.

Chinese authorities are investigating the couple — Kevin Garratt and his wife Julia Dawn Garratt — for suspected theft of military and intelligence information and for threatening national security.