According to a note released by the Scottish bank, it was informed earlier in the year that the acquisition would be looked into by the regulatory body.
The deal, which was completed in 2007 for $98 billion, saw RBS - along with Santander and Fortis - outbid Barclays in order to take control of the Dutch financial institution.
Following the acquisition RBS launched a rights issue in order to raise further funding and was forced to give up a majority stake to the government in exchange for a $33 billion bailout last year.
"In April 2009 the FSA notified the group that it was commencing a supervisory review of the acquisition of ABN AMRO in 2007 and the 2008 capital raising," RBS said in a statement, adding that it is "co-operating fully" with the body.
The government's bailout of RBS means that around 70 per cent of the bank is currently owned by the UK taxpayer.
Written by Claire Archer