The boss of HSBC has been accused of trying to downplay its role in a crackdown on Hong Kong dissidents by saying he had no choice but to shut down their bank accounts.

Noel Quinn told pro-democracy politician Ted Hui in an email that HSBC was forced to freeze the activist's accounts following an order from Hong Kong police.

In the message - highly unusual for a bank boss - he said: "I regret that HSBC is not able to operate your bank and credit card accounts."

Tom Tugendhat, chair of the Commons foreign affairs committee, described Mr Quinn's letter as "extraordinary". Mr Tugendhat said: "He’s clearly defending his actions by denying responsibility".

Dozens of pro-democracy figures have been arrested in Hong Kong since a controversial national security law was imposed by Beijing last summer. Banks can be ordered to freeze the accounts of anyone deemed a threat.

The law criminalises anti-government movements in the former British colony and has been widely condemned, but was backed by HSBC and fellow London-based lender Standard Chartered.

In a Facebook post, Mr Hui called on British MPs to investigate HSBC.

The campaigner said he had never been questioned about any transactions and asked whether the lender had followed professional procedures, or whether it had simply found his accounts suspicious "because the police said so".

Mr Hui said: "The international bank has put its customer service on the pillar of shame in its political toadyism."

The Communist leadership in China is under growing pressure for stamping out freedom in Hong Kong and putting Uighur Muslims in the region of Xinjiang into forced labour camps.

Banks such as HSBC and Standard Chartered that have long acted as a financial bridge between East and West are now caught in the crossfire. Its bosses have avoided public comments on the deepening crisis.

Mr Hui had his bank accounts frozen over a month ago, when he fled Hong Kong for the UK as he feared jail in the face of a national security law investigation. He was one of the 15 former pro-democracy lawmakers who resigned from Hong Kong’s Legislative Council in November.

A spokesman for HSBC said: "we are unable to comment on specific accounts. HSBC is required to comply with the law in every jurisdiction in which we operate".