Fortis shares lost almost a fifth of their value during Thursday trading on the Euronext Stock Exchange and closed at €10.26.
"Sack the board"
"Fortis shares are among the most popular with Belgian investors and you shouldn't be reckless when so many people have given you their confidence."
Hein Lannoy of the Bank and Financial Services Commission told the VRT that people with their savings banked at Fortis should not panic.
"You don't need to run to the bank to withdraw all your money and there is absolutely no need to panic."
The Bank and Financial Services Commission stresses that Fortis is solvent and that the bank complies with all the Belgian and international solvency ratios for financial institutions.
Meanwhile, staff at Fortis Bank branches across Belgium are having to answer a torrent of questions from concerned savers and investors.
Fortis believes it was right
Mr Lippens said that Fortis was right to float new stock despite having first said that it had no plans to do so.
Meanwhile, the financial services group has said that the position of its CEO Jean Paul Votron is safe.