HSBC clients with accounts at its Swiss private bank in Geneva are being targeted in a widening probe by the UK’s HM Revenue & Customs into alleged tax evasion, Bloomberg reported, citing two unnamed sources.

The tax authority will write to an additional 4,500 clients of HSBC, giving them the choice of making a full disclosure or face investigation, the report said.

About 800 customers have already been sent so- called Code of Practice 9 letters informing them that their tax affairs over the past 20 years will be investigated, the report continued.

The report said HMRC is devoting more staff to the investigation. The tax authority received the data from its French counterparts after Herve Falciani, a former software technician at HSBC in Geneva, stole details on at least 24,000 accounts.

The bank did not comment to this publication on the matter at the time of going to press. HMRC declined to comment.

Last month, Alexandre Zeller, chief executive of HSBC’s Swiss unit, reportedly said in August that first-half outflows from European clients had been “considerable”, in part due to the data theft incident.

In August, the UK and Switzerland agreed to iron out tax disputes by a deal under which customers either make a declaration to HMRC or pay a withholding tax that takes into account previous unpaid taxes. Switzerland also reached a similar recent deal with Germany. Meanwhile, the US has also been investigating a number of Swiss and other foreign banks in its crackdown on alleged tax evaders.