An ordained clergyman, who has written extensively about morality and banking, Green was one of the few British bank bosses to come through the financial crisis with his reputation largely intact.

But the leaking of a vast array of detail about how HSBC's Swiss-based private bank allegedly helped some wealthy clients to dodge taxes has raised questions how Green apparently failed to spot serious control and compliance failings in his own back yard.

Opposition politicians in Britain are demanding a response from Green who was chief executive and subsequently executive chairman when much of the alleged evasion occurred.

The scandal has put Prime Minister David Cameron on the defensive. He made Green a trade minister in 2010 and put him in the British upper house of parliament.

After the leak of its Swiss bank's customer list, HSBC said on Sunday that its Swiss arm had not been fully integrated after its purchase in 1999, allowing significantly lower standards of compliance and due diligence to persist.

So far, Green, 66, has declined to comment.

Cerebral and softly-spoken, Green is far removed from the brash, wheeler-dealer image of modern-day bankers.

His first job after leaving Oxford University was in a centre for recovering alcoholics in London's East End. He then moved on to the British civil service and later management consultancy at McKinsey before joining HSBC in 1982.

Green worked his way up through the ranks and was appointed to the board of HSBC Group in 1998 with responsibility for investment banking, asset management and private banking. Two years before, Green, an ordained Anglican priest, published a book about how to serve God and Mammon.

Appointed chief executive in 2003, Green oversaw the integration of U.S. consumer finance group Household International, an acquisition which eventually cost the bank billions of dollars in writedowns on its subprime mortgages.

As the scale of Household's problems emerged one major investor said at the time Green had been "asleep at the wheel".

The Household deal was one of a string of acquisitions undertaken by then chairman John Bond, who wanted to shake-up HSBC's traditionally cautious approach to banking and give it a U.S. presence to match its strong Asian and European businesses.

Bond, who later went on to chair British telecoms firm Vodafone (>> Vodafone Group plc) and mining group Xstrata (>> Glencore PLC), also oversaw the purchase of the Swiss bank at the centre of the tax storm.

Former bankers at HSBC say the bank lost its way with deals such as Household and Edmond Safra's Republic National Bank of New York and its Swiss arm in 1999, hiring people with less discipline and not keeping control of what was going on in the new corners of the organisation.

When the financial crisis struck in 2008, the billions of pounds of taxpayer money spent bailing out British banks such as Royal Bank of Scotland (>> Royal Bank of Scotland Group plc) and Lloyds (>> Lloyds Banking Group PLC) destroyed the reputations of many of their executives.

But Green made it through largely intact. HSBC did not need a state-funded rescue.

Chairman of HSBC from 2006 until 2010, Green also found time in 2009 to publish a book about individual responsibility entitled ‘Good Value: Reflections on Money, Morality and an Uncertain World'.

When appointed trade minister in 2010, business secretary Vince Cable, a member of Cameron's junior coalition partners, the Liberal Democrats, said Green was: “One of the few to emerge with credit from the recent financial crisis, and somebody who has set out a powerful philosophy for ethical business.”

But in the years since his retirement, Green's reputation has started to come under scrutiny.

In 2012, HSBC had to pay a record $1.9 billion fine after U.S. authorities said it had become the preferred financial institution for drug traffickers and money launderers between 2006 and 2010, the years of Green's chairmanship.

(Reporting by Carmel Crimmins. Editing by Jane Merriman)

By Carmel Crimmins