The shares are being sold to help finance the $98 billion takeover of ABN Amro alongside Royal Bank of Scotland and Santander.
Merrill is set to earn base commission of about 42 million euros for underwriting the sale as well as a fee for advising the group on the take over and Alan Webborn, an analyst at Societe Generale SA in London, told Bloomberg that it was a big commitment.
"Selling paper in the banking industry at any price isnât easy right now," he said.
Fortis began selling shares at $21 today, 44 per cent below the closing price on September 20th.
The offering is said to be the second-biggest sale of shares in dollar terms after Industrial & Commercial Bank of China Ltd made a $22 billion initial public offering lat year.