Banking scandals have become familiar to the financial community, but even the gaff-prone investment banking sector is reeling at accusations that British banks have helped Russian gangsters launder money in a long-standing scheme now facing scrutiny.

According to The Guardian, HSBC plc (HSBC), Barclays plc (BCS) and The Royal Bank of Scotland Group plc (RBS) are among the banks "facing questions" over an international conspiracy to launder almost $740 million of criminally acquired funds for mafia clients.

The alleged scheme lasted four years between 2010 and 2014, when bankers turned a blind eye to massive amounts of money going from Russia to accounts based outside the former Soviet nation. According to one unnamed person involved in the scandal, the funds were "obviously either stolen or with criminal origin."

One investigator in the case believes as much as $80 billion may have been laundered thanks to the help of British banks.

As many as 500 people may have been involved, including Russian "oligarchs, Moscow bankers and figures working for or connected to the FSB," the Federal Security Service of the Russian Federation. The FSB has been widely criticized for its alleged investigations and possible murders of political opponents of Vladimir Putin. Former FSB officers have accused the organization of using a spy network around the world to attack Russian enemies.

Despite criticisms and the country's dedication to NATO in limiting Russia's influence in Europe, British government tax programs have encouraged many Russian oligarchs, FSB officials and alleged Russian organized crime leaders to purchase property and maintain residences in London. As a result, a number of internal political scandals have stretched to involve the capital of the U.K. For instance, in 2006, former FSB officer Alexander Litvinenko was poisoned by a rare radioactive isotope, polonium-210, resulting in his death. One of the supposed murderers of the Russian national, another Russian named Boris Berezovsky, was found dead in England seven years after Litvinenko's death.

U.K. banks with international reach have been extremely supportive of Russian clients based in Britain, offering special concierge services for ultra high net worth individuals. Foreign banks in the U.K. have also offered special services to Russian and other ultra-wealthy customers based in the country. For instance, when Cyprus's banking system closed and instigated a levy on all deposits within the country, Cyprus bank branches in England remained open for business, offering Russians with accounts in Cyprus the opportunity to withdraw their funds and avoid the penalty.

British banks and financial regulators in the United Kingdom have remained silent on the apparent ongoing investigation.