Noel Quinn, HSBC chief executive, personally wrote to a Hong Kong pro-democracy activist to say that the bank was legally obliged to freeze his account, in a sign of the tightrope that corporations must walk to operate in the Chinese territory.

Ted Hui, a former lawmaker who fled to the UK and was investigated by Hong Kong authorities for allegedly violating Beijing’s tough new national security law, had his and his family’s accounts frozen by HSBC in December on police instructions. Police initially said as much as HK$850,000 ($110,000) had been frozen, but Mr Hui said the figure was closer to HK$200,000.

HSBC, which derives a significant proportion of it profits in Hong Kong, has infuriated the US and UK by openly backing China’s security law that had been denounced by western governments. Thousands of foreign companies based in Hong Kong must navigate an increasingly tricky corporate environment amid a tightening Chinese crackdown on the former British colony and heightened Sino-US tensions.

Police have in recent months widened the crackdown on the pro-democracy opposition involved in the Hong Kong pro-democracy protests of 2019, including by targeting activists’ commercial and financial interests.

Mr Quinn’s intervention comes after several UK lawmakers in parliament criticised HSBC’s treatment of Mr Hui and after Hong Kong pro-democracy activists said the incident undermined their trust in the bank.

Richard Harris, veteran Hong Kong investor and founder of Port Shelter Investment Management, said the “extremely unusual” intervention by Mr Quinn was an attempt to calm the waters 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 said in the email sent to Mr Hui last Monday that police had instructed them to act. “[He] explained that the bank was ‘not able to operate’ my bank and credit card accounts and ‘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 approach, which he 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 has to obey the laws of the jurisdictions in which it operates.

Mr Hui is facing charges over his participation in the 2019 protests that started over a bill that would for the first time allow suspects to be extradited to mainland China.

The introduction of the security law in June — seen as China’s response to the demonstrations — heralded a clampdown on the city’s opposition, and mass arrests.

Mr Hui fled the country along with his family last year with the assistance of Danish lawmakers.

Authorities have accused him of violating bail, and since said they were investigating whether he had violated the security law, which targets subversion, foreign interference and collusion and at its most severe can carry a life sentence.

Police said they froze Mr Hui and his family’s accounts because they were investigating him for money laundering over a crowdfunding operation he was tied to that also involved his family’s accounts.

The pro-democracy 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, who has previously accused Hong Kong authorities of using the law to suppress “the voices of opposition”, has been contacted for comment.