The Financial Conduct Authority is investigating a raft of currency traders' private accounts to see if they shifted their own money ahead of client orders and before allegedly attempting to manipulate the FX market.
According to media reports, citing unnamed sources, the FCA is has asked several banks to probe into whether traders used undeclared personal accounts to cash-in on FX trading ahead of placing orders for customers.
The daily $5tn (£3.1tn, €3.7tn) currency market is the largest in the financial system and is pegged to the value of funds, derivatives and financial products.
Morningstar estimates that $3.6tn in funds, including pension and savings accounts, track global indexes.
A number of banks around the world have revealed that they have launched internal reviews into their FX trading procedures, after America's Department of Justice and the Federal Bureau of Investigation launched a criminal investigation into whether the world's biggest banks attempted to manipulate the currency markets.
Switzerland's Financial Market Supervisory Authority (Finma) is also looking into whether FX market rigging has occurred.
At the beginning of November, Lloyds Banking Group confirmed that it is reviewing its currency trading processes.
RBS said in a statement that "we are currently considering processes around the benchmark service."
On 23 October, an RBS sales team sent an email to clients to say that it is reviewing how it trades in the minutes before key FX benchmarks are set.
A day before RBS' announcement, Deutsche Bank revealed that its balance sheet was hit by billions of euros of litigation costs which subsequently led the German lender to report a near 100% drop in profits.
Deutsche Bank has allegedly spent millions of dollars going through traders' emails and chat sessions looking for specific dates, phrases and keywords in a bid to root out evidence of wrongdoing.
Traders' personal accounts have to be declared to the bank, and usually to a trader's line manager, alongside each individual trade.
Meanwhile, investment banks' rules on private accounts vary widely from an outright ban to more relaxed guidelines.
For example, UBS has forbidden any of its traders from owning a personal spread betting account while other banks have more relaxed rules.
The use of traders' personal accounts fell into the spotlight in 2012 after the UBS rogue trader, Kweku Adoboli, accused of losing billions of pounds in illicit deals was relying on a number of pay-day loan companies at the time of his arrest.
The court heard that even though Adoboli earned £360,000 in 2010, the year of his arrest, the trader also racked-up substantial personal losses in the months before his activities were uncovered.
After losing £123,000 of his own money through spreadbetting on the future movement of the financial markets, via an account with IG Index, Adoboli made a number of payments from his current account to advance pay-day loan companies, including Wonga.com, Payday UK, Moneybox, Wageday Advance, which charge interest of several thousand percent on their loans, the court was told.