The US could be heading for a legal showdown with the Swiss Government after it emerged that a senior UBS banker who is charged with conspiracy to evade taxes is in Switzerland.

Raoul Weil, the chairman and chief executive of global wealth management at UBS, was indicted by American authorities earlier this week after allegedly helping as many as 20,000 people avoid income tax by hiding their assets.

Last night, a spokesperson for the US Department of Justice (DoJ) told The Washington Post that prosecutors believe Mr Weil is in his native Switzerland. If that is true and Mr Weil refuses to return voluntarily to the US, prosecutors could face an uphill struggle to bring him to trial.

Although Switzerland has a bilateral extradition treaty with the US, the treaty includes a get-out clause allowing Switzerland to refuse to send its own citizens abroad to face criminal charges.

Mark Spragg, an extradition expert at Jeffrey Green Russell, a law firm, said it was unlikely that Swiss authorities would cooperate with an extradition request for Mr Weil.

If that was to occur, efforts to prosecute Mr Weil would be further frustrated as he cannot be tried in absentia.

In a statement, Mr Weil’s lawyer said that the charges against his client are "totally unjustified" and said that his client "fully intends to fight this indictment". Neither Mr Weil nor his lawyer could be reached for further comment.

The DoJ has accused Mr Weil of the broad offence of conspiracy to evade tax, rather than the narrower offence of tax evasion. This makes the prosecutors’ burden of proof much easier as they need only show that Mr Weil took steps that furthered a broader conspiracy.

Under American law, conspiracy to evade tax carries a maximum sentence of five years in jail and a $250,000 fine.

If Mr Weil chooses to remain in Switzerland and is not extradited, he could face permanent travel restrictions. The US Government could produce a warrant for his arrest and circulate it to Interpol, the International Criminal Police Organisation.

UBS, which said Mr Weil has stepped down from his duties at the bank, declined to comment on his location.

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