The U.K. tax agency is investigating almost 800 British holders of HSBC Holdings Plc (HSBA) accounts in Switzerland on suspicion of tax fraud, the Sunday Times said.

The tax situation of more clients is likely to be probed as the agency examines records of 7,000 customers stolen by a former HSBC employee in Switzerland, the newspaper said, citing unidentified accountants and officials at HM Revenue & Customs.

Clients in recent months have received a letter notifying them they are “suspected of serious tax fraud,” the Times said. The probe may raise hundreds of millions of pounds in revenue, the newspaper said.

Those affected cannot benefit from the recent offshore tax agreement between the U.K. and Switzerland, the newspaper added. The Times cited HSBC as saying the bank doesn’t condone tax evasion and seeks to comply with the law in all countries it operates in.