A former UBS banker has been charged by US prosecutors with helping approximately 150 wealthy Americans evade up to $500 million in income taxes, according to published reports.

The banker, Christos Bagios, was arrested last month. He currently works for Credit Suisse as a senior relationship manager in the US, according to the Wall Street Journal. The complaint concerns his actions while working at UBS.

Spokespeople at Credit Suisse and UBS declined to comment when contacted by this publication. A lawyer for Bagios couldn’t be reached for comment before this story went to press.

Last month US authorities also indicted four bankers on charges of helping US clients hide their assets in Swiss accounts. Three of them were formerly at Credit Suisse and one is still at the bank, according to the WSJ. The four live outside the US and have not been arrested, it said.

"We are cooperating with the authorities in their investigation of these individuals. Credit Suisse is not a target of the investigation," a spokesperson for Credit Suisse in New York told this publication when that development was first reported.

The latest developments come in the midst of a widening crackdown by US authorities on offshore accounts. They also come two years after UBS became embroiled in a large and damaging tax evasion case over its alleged role in assisting wealthy US citizens to stash money in offshore centres. UBS has since ceased to provide offshore banking to US clients.