New York Attorney General Andrew Cuomo will file suit against Merrill Lynch & Co. tomorrow morning if it doesn’t reach an agreement by the end of today with the investment bank regarding its marketing of auction rate securities, its office said.

“Enough is enough,” Mr. Cuomo told the AP in an interview. “Delay doesn’t work as a tactic. I want them to come in quickly and resolve this expeditiously.”

Meanwhile, the Massachusetts Secretary of the Commonwealth issued a press release saying it had struck an agreement with Merrill, under which the investment bank would buy back its auction rate securities at par starting Oct. 15 from customers with less than $3 million on deposit.

On or after Jan. 15, the bank would acquire all illiquid auction-rate securities at par from all other clients with deposits of $100 million or less, according to the agreement.

Merrill Lynch, which has roughly $12 billion in auction rate securities, declined to comment on the deal with the Massachusetts Commonwealth, casting some doubt as to whether it is on board with the agreement.

The AG’s office reached settlements totaling $42 billion with five major Wall Street banks in recent weeks. Earlier this month, Merrill Lynch told Mr. Cuomo’s office it would buy back its client’s illiquid auction rate securities, although the plan was voluntary and set no specific timetable.

Mr. Cuomo said he had given Merrill Lynch a five-day warning that it must set a timetable to buy back the securities and pay a fine for marketing them as safe investments to clients before it would take legal action.

“The main point is the timing of how quickly they offer the buybacks to investors,” Mr. Cuomo told the AP. “We want that done quickly, and they had a voluntary time frame that was not acceptable.”

The $330 billion auction rate securities market collapsed in February. The instruments resemble long-term corporate debt, but the interest rates are reset at auctions, sometimes as frequently as every seven days. Banks touted the securities as similar to cash.