HONG KONG (Reuters) -Shares of Hong Kong beverage maker Vitasoy tumbled 12% on Monday after a worker sent around a memo offering condolences to the family of a colleague who stabbed a Hong Kong policeman, prompting social media users in China to call for a boycott of the company.

Vitasoy said in a statement on social media platform Weibo on Saturday that a staff member had circulated an internal memo that was widely shared online, describing it as "extremely inappropriate" and without authorisation. It added that the company reserved the right to take legal action.

The employee's memo offered condolences to the family of a 50-year-old Vitasoy worker who had stabbed a police officer, 28, and then killed himself on Thursday, the anniversary of the former British colony's return to Chinese rule.

Police and security officials have described the stabbing as a terrorist attack by a lone wolf, saying a computer they seized from his home showed he had been "radicalised." Police did not provide further details about his alleged radicalisation.

The officer's condition has improved to serious from critical.

The memo triggered a flood of online calls for a boycott of the company, which gets two-thirds of its revenue from mainland China.

Mainland actors Gong Jun and Ren Jialun, who have worked with Vitasoy, plan to end ties with the company, according to the Global Times tabloid.

Hong Kong authorities warned on Sunday that advocating for people to mourn for the attacker was no different from "supporting terrorism" and criticised parents who took children to mourn him.

(Reporting by Donny Kwok; Editing by Lincoln Feast and Edwina Gibbs.)