Britain’s largest bank has apologised following claims it helped rich clients dodge taxes, which have ignited a huge political scandal in the country.

HSBC Chief executive Stuart Gulliver says firestorm has been a painful experience, and that the allegations deal with historical practices that no longer occur.

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Gulliver said on Sunday an ad had been placed in British newspapers stating HSBC had “no appetite” now to help clients hoping to avoid paying taxes.

He also said in a letter addressed to clients and staff that the bank “must show we understand that the societies we serve expect more from us. We therefore offer our sincerest apologies.”

HSBC has been shaken by leaked documents, most from around 2005-2007, that show it helped wealthy customers evade taxes.

The political fallout of the allegations have claimed the job of former HSBC boss Stephen Green.

Mr Green has resigned from TheCityUk, a lobby group for the financial services industry.

—with AAP.