Noel Quinn, HSBC chief executive, personally wrote to a Hong Kong pro-democracy activist to express regret after the bank froze his bank account, in a sign of the challenges that companies are facing in the Chinese territory.

HSBC froze the bank accounts of Ted Hui, a former lawmaker who fled to the UK and is under investigation by Hong Kong authorities for allegedly violating a new national security law, and those of his family in December. Police initially said as much as HK$850,000 (US$110,000) had been frozen, but Mr Hui said the figure was closer to HK$200,000.

The bank, which derives a significant proportion of its profits in Hong Kong, has infuriated the US and UK by openly backing the security law, which Beijing directly imposed on the city in June. But thousands of foreign companies based in the territory are navigating an increasingly tricky corporate environment in the midst of a tightening crackdown on the former British colony and heightened Sino-US tensions.

Police have in recent months targeted the commercial and financial interests of pro-democracy activists involved in the Hong Kong pro-democracy protests of 2019.

“I regret that HSBC is not able to operate your bank and credit card accounts,” Mr Quinn wrote in the email sent to Mr Hui last Monday and seen by the Financial Times.

The HSBC CEO also apologised over confusing messages from the bank.

Mr Quinn’s email came after several UK lawmakers criticised HSBC’s treatment of Mr Hui and pro-democracy activists said the incident undermined their trust in the bank.

Richard Harris, a veteran Hong Kong investor and founder of Port Shelter Investment Management, said the “extremely unusual” intervention was an attempt to sooth concerns among any locals who may fear for the security of their deposits.

“If you send something to a public figure you could foresee it would be used publicly,” Mr Harris said. “He’s hoping to send a message of ‘Don’t worry, this is [a] particular situation and this is no fault of ours’.”

Mr Quinn told Mr Hui that police had instructed the bank to act. “[He] explained that the bank ‘had no choice’ but . . . to take action following notification from the Hong Kong Police,” Mr Hui wrote on his Facebook page.

Mr Hui rejected Mr Quinn’s argument, which the former lawmaker said was the first proper explanation from the bank since the accounts were suspended. “Any institution is an accomplice of human right infringement[s] where it acts . . . against due procedures with the effect of oppressing freedom,” Mr Hui said.

HSBC said it would not comment on matters relating to specific accounts, but the bank has previously said it must obey the laws of the jurisdictions in which it operates.

The introduction of the controversial security law heralded a clampdown on the city’s opposition and waves of mass arrests. Mr Hui was on bail following charges related to his role in the 2019 protests when he fled with his family last year with the assistance of Danish lawmakers.

Authorities have accused him of violating bail and said they were investigating whether he had contravened the security law, which targets subversion, foreign interference and collusion.

Police said they froze Mr Hui and his family’s accounts because he was under investigation for money laundering in connection with a crowdfunding operation, which also involved his family’s accounts.

The activist said the crowdfunding campaign, which was set up to support a legal effort against alleged police brutality, had nothing to do with his family.

Mr Hui said Hong Kong authorities were using the law to suppress opposition.