According to the investment bank, Ms Sergeant is departing to pursue other interests and a replacement for her will be sought.
She initially began her career within the sector at the Bank of England in 1974 before joining the Financial Services Authority (FSA) in 1997.
Ms Sergeant is thought to be one of the frontrunners to take on a management role at any of the new regulatory bodies set up in the wake of the dismantling of the FSA.
In a statement quoted by Bloomberg, the departing post holder said: âNow is the right time for me to seek new challenges.â
Her departure from the bank follows the recent announcement that AntÃ³nio Horta-OsÃ³rio will take over the role of chief executive officer (CEO) from March 1st.
The new post holder will succeed outgoing CEO Eric Daniels and joins from Santander UK where he has been working in a similar role since 2006.
By Jim Ottewill