HSBC Holdings PLC could be open to criminal charges in the U.K. over the role played by its Swiss private bank in a massive tax-evasion scheme, according to Lord Ken Macdonald, who served as director of public prosecutions in England and Wales between 2003 and 2008. He said a decision to forgo prosecution made by the British tax-collection agency called Her Majesty’s Revenue and Customs, or HMRC, was “seriously flawed,” BBC News reported Sunday.

Macdonald made his observations in a legal opinion prepared for the consumer organization SumOfUs, which is based in New York and incorporated in Washington.

“It seems clear, from the evidence we have seen, that there exists credible evidence that HSBC Swiss and/or its employees have engaged over many years in systematic and profitable collusion in serious criminal activity against the exchequers of a number of countries,” BBC News quoted Macdonald as saying in his opinion.

“The corporate and wholesale nature of HSBC’s Swiss’ apparent involvement in what amounts to grave cross border crime makes it all the more obvious that the relevant evidence, once it came the attention of HMRC, should have been the subject of urgent and sustained criminal investigation,” BBC News quoted Macdonald as saying.

Neither HSBC nor SumOfUs replied to International Business Times email messages requesting information before this article’s publication.

HSBC has faced a lot of scrutiny over its Swiss private bank’s past operations this month since documents about them were leaked to the International Consortium of Investigative Journalists via the Paris-based newspaper Le Monde. The leaked files are associated with accounts holding more than $100 billion.

HMRC has had evidence about the tax-evasion scheme for almost five years, but has taken no action in the case, BBC News reported.

The U.K.’s Financial Conduct Authority indicated last week it would look into the matter, and the Bank of England and the Serious Fraud Office suggested they may do likewise, BBC News said.

HSBC published in several British newspapers last weekend full-page advertisements featuring a letter of apology over the past conduct of its Swiss private bank.

In the letter, HSBC Group CEO Stuart Gulliver acknowledged: “The media focus has  been on historical events that show the standards to which we operate today were not universally in place in our Swiss operations 8 years ago. We must show we understand that the societies we serve expect more from us. We therefore offer our sincerest apologies.” It directed those who would like more information about the matter to a document delivered via this online site.