HONG KONG, July 4 (Reuters) - Beverage maker Vitasoy has become the latest target of Chinese netizen calls for a boycott after an employee circulated a memo online offering condolences to the family of a worker who had stabbed a Hong Kong police officer.

In a statement on the Chinese social media platform Weibo on Saturday, Vitasoy said a staff member had circulated a memo that it described as "extremely inappropriate" without authorisation, and the company reserved the right to take legal action.

The 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 the former British colony's return to Chinese rule, media outlets reported.

Hong Kong's Secretary for Security Chris Tang has described the assault as a "lone wolf" terrorist attack.

"What this employee wrote should not have been made public and should not have been published internally," Vitasoy said.

"Vitasoy Group sincerely apologises for any troubles or grievances this has caused. We support Hong Kong's long-term prosperity, stability and development."

The worker's memo triggered a flood of online calls for a boycott of Vitasoy, which gets two-thirds of its revenue from mainland China.

The hashtag "#Vitasoygetoutofthemainland" had garnered almost 100 million views by Sunday.

Mainland actor Gong Jun, who previously endorsed a Vitasoy lemon-flavoured drink, announced late on Friday he was ending commercial cooperation with the company, said Global Times, a tabloid published by the ruling Chinese Communist Party's official People's Daily newspaper.

His announcement followed that of another mainland Chinese actor, Ren Jialun, who said he was also ceasing co-operation with Vitasoy, the newspaper added.

Fashion retailer H&M said on Thursday its sales took a hit in China after its concerns over alleged human rights abuses in Xinjiang led to a social media-inspired boycott by shoppers.

(Reporting By Anne Marie Roantree in Hong Kong and David Kirton in Shenzhen; Editing by William Mallard)