The service, which was founded by and remains under the control of New York mayor Michael Bloomberg, has been part-owned by the bank for the past 20 years.
However, recent pressure on Merrill's balance sheet due to the credit crunch is thought to have brought about the potential sale - with the 20 per cent stake now worth an estimated $6 billion.
Talks are understood to be ongoing between the bank and the news agency.
Other Wall Street firms facing similar pressures have already announced sales of liquid assets in order to shore up their capital ratios.
Citigroup's chief executive Vikram Pandit, for example, said earlier this year that the bank needed to "get fit" by selling off up to $500 billion of assets before 2011.
Total write-downs relating to the ongoing global financial crisis in the financial sector since last summer are understood to exceed $300 billion.