Ms Chow was jailed in 2020 for taking part in the anti-government protests of 2019, and was released in 2021.

She is still under investigation for "collusion with foreign forces to endanger national security".

Local media reported that Hong Kong police "strongly condemns" the 27-year-old's actions.

It also urged her not to "take the road of no return by remaining a fugitive for the rest of her life", Hong Kong broadcaster TVB said in a report on Monday.

In two Instagram posts published on Sunday to mark her 27th birthday, Ms Chow revealed that she was admitted by a university in Toronto earlier this year. She left for Toronto in mid-September.

But in order to get her passport back, she had to go on a police-escorted trip to mainland China in August with five police officers - a trip she had no right to refuse.

"I felt as though I was under surveillance the whole time," she wrote.

She said she was shown an exhibition of China's achievements since the reform and opening up of the country since the late 1970s, as well as the headquarters of the technology firm Tencent where she was asked to pose for photos.

"If I stay silent, those pictures might one day become evidence of my "patriotism" - that fear is so tangible," she wrote.

Ms Chow said when she returned to Hong Kong, she was also told to sign letters expressing remorse for all her past political actions, and also to thank the police for organising the trip, so that she could learn of "the motherland's marvellous developments".

Ms Chow was expected to report to the police again later this month as she is still under investigation in connection to pro-democracy media tycoon Jimmy Lai's trial on allegations of "collusion with foreign forces" to endanger China's national security. She has not yet been charged.

She said her decision was made after much consideration of "Hong Kong's situation, my personal safety, my physical and mental health".

"Maybe I won't return to Hong Kong for the rest of my life," Ms Chow added.

Ms Chow was one of the most prominent faces of the city's pro-democracy movement and was even nicknamed "the real Mulan", in reference to the legendary Chinese heroine who fought to save her family and country.

In 2020, she was featured on the BBC 100 Women list, which names 100 influential and inspirational women around the world every year and tells their stories.

Authorities in Hong Kong have cracked down on pro-democracy activists in the city since anti-government protests broke out in 2019, culminating in the implementation of the controversial national security law in 2020.

By Vicky Wong
BBC News