Shares in Hong Kong carrier Cathay Pacific have fallen after it warned staff that they could be fired if they "support or participate in illegal protests".
The warning follows new regulations imposed by China's aviation regulator requiring Cathay Pacific to submit manifests of staff on flights to the mainland or through its airspace.
China told the airline that staff involved in the protests that have gripped Hong Kong for more than two months would be banned from flights to the mainland.
The airline has already said it will comply with those regulations, citing the importance of its business in China and the requirement to adhere to local rules.
But in a message to staff, chief executive Rupert Hogg reiterated that Cathay Pacific employees would also face "disciplinary consequences" if they get involved in the pro-democracy protests.
"Cathay Pacific Group has a zero tolerance approach to illegal activities. Specifically, in the current context, there will be disciplinary consequences for employees who support or participate in illegal protests," Mr Hogg wrote.
"These consequences could be serious and may include termination of employment."
Mr Hogg also specifically warned employees not to support or participate in a new protest at Hong Kong airport called today.
He reminded staff that the "actions and words of our employees made outside of working hours can have a significant effect on the company".
Shares in the airline were 4.4% lower following publication of the letter.
The protests in Hong Kong have infuriated Beijing and left Cathay Pacific in a difficult position.
It has already suspended a pilot who has been accused of rioting after allegedly participating in the Hong Kong protests.
It said on Saturday that it had fired two airport ground staff, without specifying why.
Local media reported that they were accused of leaking the travel details of a Hong Kong police football team that was travelling to the mainland.