The report listed the banks as Barclays, NatWest, UBS, and HSBC. It said this had raised questions about their commitment to tackling financial crime.
The watchdog said the banks had been helping politicians to keep funds stolen from their countries. This was in spite of their indictment in the laundering of funds stolen from Nigeria by former dictator, Gen. Sani Abacha, in the 1990s.
It also outlined how both former governors and their suspected accomplices moved money from Nigeria to these banks.
The report demanded an end to the acceptance of â€˜dirty moneyâ€™ by UK banks.
â€œWithout access to the international financial system, it would be much harder for corrupt politicians from the developing world to loot their national treasuries or accept bribes. By taking money from such customers, British banks are fuelling corruption, entrenching poverty and undermining international development assistance.
â€œGlobal Witness has found that Barclays, HSBC, RBS, NatWest and UBS held accounts for two former Nigerian state governors, Diepreye Alamieyeseigha of Bayelsa State and Joshua Dariye of Plateau State. These men funnelled dirty money into the UK, spending their ill-gotten gains on sustaining a luxury lifestyle, in stark contrast to the poverty of ordinary Nigerians.
â€œThe Nigerian government took the governors to court in London to recover their illicit assets. The cases were successful and British judges ordered their UK assets to be returned.â€
The report said the banks were supposed to have tightened up their procedures since the Abacha saga, adding that its investigation revealed that they had not done enough.
It urged the UK regulator, the Financial Services Authority, to â€œdo much more to prevent banks from facilitating corruption. As yet no British bank has been publically fined, or even named, by the regulator for taking corrupt funds, whether willingly or through negligence.â€
It said this was in â€˜stark contrastâ€™ to the United States where banks had been fined hundreds of millions of dollars for handling dirty money.
â€œThe FSA is due to be abolished next year. Whichever organisation takes over regulating the banking sector must take corruption seriously.
â€œBanks that accept corrupt funds should be named, and if it is found that they acted negligently, heavily fined. Banks also needed to be provided with more information about how to spot corrupt money,â€ the report said.
Global Witness noted that the UK had centered its aid to poor countries on budget cuts while its banks were sustaining practices that were inimical to development assistance.
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