A senior HSBC executive has reportedly been charged in the US for his alleged role in a conspiracy to rig international currency markets.Mark Johnson, HSBC's global head of foreign exchange cash trading in London, was arrested in New York on Tuesday, according to media reports.Mr Johnson is expected to appear in a federal court in Brooklyn later on Wednesday, according to Bloomberg which first reported the news.HSBC has so far declined to comment.In May 2015, The Justice Department investigation into foreign-exchange rigging at global banks led four banks to plead guilty to charges including manipulating the foreign exchange market.HSBC was not part of those criminal charges, but it was one of six banks fined by UK and US regulators over their traders' attempted manipulation of foreign exchange rates in November 2014.HSBC reported last August that it had set aside $1.3bn for possible settlements to cover costs related to ongoing legal investigations.About 40% of the world's currency dealing is estimated to go through trading rooms in London.The massive market, in which $5.3 trillion worth of currencies are traded daily, dwarfs the stock and bond markets.There is no physical forex marketplace and nearly all trading takes place on electronic systems operated by the big banks and other providers.Daily "spot benchmarks" known as "fixes" are used by a wide range of financial and non-financial firms to, for example help value assets or manage currency risk.