The prospect that Merrill Lynch could anchor one of the towers planned for ground zero is prompting the Port Authority of New York and New Jersey to give developer Larry Silverstein more time to build.

The Port Authority’s board of directors approved extensions of six months and four months for the completion of towers 3 and 4, respectively, to allow Silverstein Properties further time to negotiate with Merrill. The firm’s current lease at the World Financial Center expires in 2013.

“Merrill offers opportunities to the site that it makes sense to provide additional time,” the Port Authority chairman, Anthony Coscia, said following the board meeting held yesterday. “Any tenant of that caliber is significant.”

The extension allows more time and flexibility to tailor the floor plate structure of Tower 3 to make the office space more amenable to a major financial service company such as Merrill, which needs open space to accommodate trading floors.

Mr. Coscia said it was premature to discuss what kind of alterations to the floor plan would be necessary to ultimately lure Merrill, but said he thought that those negotiations could be fast-moving.

“Our understanding is that they are willing to do this very, very quickly,” Mr. Coscia said.

With the extensions, Tower 3 is now expected to be completed midway through 2012, while Tower 4 is scheduled to be completed in the spring of 2012.

The president of World Trade Center Properties, Janno Lieber, said he expected those dates to be met.

“This extension gives us the time to design and construct a modified building foundation that could accommodate Merrill’s specialized requirements,” Mr. Lieber said in a statement.

Last year, Merrill was set to move its offices to the Hotel Pennsylvania in Midtown, but that deal fell through due to complications tied to the departure of a former chief executive officer, Stanley O’Neal.

The Port Authority’s board yesterday officially appointed Christopher Ward as the new executive director of the Port Authority. In his first comments to the press, Mr. Ward was noncommittal on the possibility of the Port Authority controlling the redevelopment of Pennsylvania Station, an idea supported by both Senator Schumer and Governor Paterson, but opposed by Mayor Bloomberg and a number of elected officials in Albany. Mr. Ward said he appreciated the vote of confidence in the Port Authority, but that any direction would have to come from Mr. Paterson.