The sale of Direct Line Group was forced on RBS by European regulators as a condition of accepting Government aid in 2008. RBS is majority owned by the state.
DLG has been preparing for the sell-off for more than a year, creating distinct positioning for its brands to increase their value to potential investors.
About 25% of shares will be offered initially, with more to follow. Analysts have valued DLG at £3bn.
RBS must sell the majority the majority of its stake by the end of 2013 and all of it by the end of 2014.