Hervé Falciani, 46, the man who leaked the Lagarde List of HSBC clients and their account information, has been arrested in the Spanish capital of Madrid on Swiss orders.

Falciani, a Franco-Italian IT man at HSBC's offices in Geneva until he 'went rogue' and tried to sell data about 130,000 clients to various governments in the hope that they would be able to use them to track down unpaid taxes, has been in a Spanish gaol before (for two months in the middle of 2012) as the result of a Swiss arrest warrant. The courts let him go on that occasion because he was not accused of activity that was criminal in Spain.

A Swiss court convicted Falciani in his absence of aggravated industrial espionage in 2015 and condemned him to a five-year prison sentence. Although he was arrested on a warrant issued in Switzerland, the international press is not entirely certain that his present arrest stems from that judgment or from some new charge. The Spanish police stopped him in the street on the way to a conference, apparently taking him by surprise.

Like many informants, Falciani appears to be no angel and all too human. The Swiss authorities say that he went to Lebanon in early 2008 with Georgina Mikhael, an HSBC work colleague who later told the press that she was his mistress, to sell the information to various banks. Having failed to do that and having then been put under surveillance by the Swiss when they heard of it, he rented a car, collected his wife and two daughters and made a dash for the French border in December 2008. The Swiss issued their first international warrant for his arrest the next year. The French, however, never extradite their own citizens (Roman Polanski being the prime example) and used his list to open some prosecutions.

In 2012 he left France. His motive for doing so was personal safety; he claims that US authorities told him that various rich people (presumably some clients of HSBC, or perhaps the banks themselves) were hiring assassins to track him down in retaliation for leaking his list and advised him either to come to America - something he was not foolish enough to do - or to go to another country whose authorities viewed the list with approval. He therefore went to Spain, whose tax authorities acquired about €300 million (£262 million) in unpaid tax from some of the 637 Spaniards on his list. Three years ago he told the London Financial Times that his wife and offpring were living outside France, under police protection. The French tax authorities have passed his files to their colleagues around the world.

Ada Colau, the mayor of Barcelona, described Falciani’s arrest and possible extradition as a scandal, telling the London Guardian: “He is a key figure in the fight against corruption who worked with the Spanish authorities to prosecute tax fraud. He also advised Barcelona city council on how to improve transparency.”

HSBC had to pay Geneva prosecutors SFr40 million (US$42.8 million) to avoid prosecution for facilitating money laundering in 2015. The subject of the averted investigation was HSBC Private Bank (Suisse), the very same unit at which Falciani worked. This unit was the centre of a political and media storm in 2015 when the in International Consortium of Investigative Journalists in Washington DC used data leaked from the bank to claim that thousands of account-holders were dodging taxes. The publicity cost the bank dear in terms of numbers of clients and assets under management. It has also, incidentally, just had to pay a fine of US$100 million to ward off US litigation that alleges that it conspired to rig Libor, the key benchmark interest rate.

Under Swiss law it is forbidden to base an investigation on stolen evidence. This is why the prosecutors raided the unit before reaching a settlement with the bank, although it is possible that actual evidence of a crime was not (or was no longer) present.