The Federal Reserve and Department of Justice are carrying out the probe, the FT said, citing unnamed sources close to the matter. The investigation comes after RBS, parent firm to private bank Coutts, volunteered information to US and UK regulators 18 months ago, the paper said.

RBS declined to comment on the story. However, it pointed out to Family Wealth Report that, in its interim report, published in early August, it said it has engaged and will continue to engage in discussions with relevant government and regulatory authorities, including in the UK and US and over issues affecting compliance with applicable anti-bribery, anti-money laundering and sanctions regimes.

The FT said a spokesperson for the Federal Reserve said it could not "comment on supervisory matters pertaining to individual institutions." A representative at the Justice Department did not respond to a request for comment.

US authorities have been examining the actions of several foreign banks. Last week, UK-listed Standard Chartered, which earns the bulk of its revenues outside the UK, agreed to pay a $340 million settlement with US authorities relating to Iran transactions. Meanwhile, HSBC, Europe's largest bank, has estimated that it faces fines of $700 million stemming from anti-money laundering failings that have been exposed by a US investigation.

In 2010, RBS agreed to pay $500 million to settle similar allegations by US federal authorities that ABN Amro, which RBS acquired in 2007, had violated US sanction laws.

In its interim results statement issued on August 3, RBS had incurred a £8.75 million ($13.8 million) fine imposed by the FSA during the first quarter of this year. (The fine was slapped on the bank for failing to take reasonable care to establish and maintain effective anti-money laundering systems and controls relating to high-risk customers, including “politically exposed persons.”)