Lower Manhattan is receiving a boost, as Mayor Bloomberg and Governor Spitzer are expected today to announce that JPMorgan Chase & Co. will build and occupy a skyscraper of about 50 stories at the edge of the World Trade Center site, according to people familiar with the plans.

The financial giant is said to be paying about $300 million to the Port Authority of New York and New Jersey for a long-term lease. It would construct a 1.3 million-square-foot office tower on the site at 130 Liberty St. after the Deutsche Bank building is demolished.

Coupled with the construction of a new Goldman Sachs headquarters in Battery Park City, to be completed in 2009, and a mostly leased 7 World Trade Center building, completed in 2006, the move by JPMorgan serves as a telling vote of confidence for downtown, an area that was by no means guaranteed a comeback after the terrorist attacks of September 11, 2001.

In the months that followed, numerous firms in the financial industry fled to Midtown and elsewhere, depressing rents and rendering the future of the area uncertain. As the economy and demand for real estate has surged, so too has the demand for office space downtown— just months ago, real estate insiders expected the site at 130 Liberty St. to become an apartment building.

“It’s great for Lower Manhattan,” the CEO of the Partnership for New York City, Kathryn Wylde, said. “JPMorgan Chase, as it’s called, is the consolidated result of four major banks that were headquartered in Lower Manhattan for many years,” Ms. Wylde added, praising its return to the financial district.

Real estate analysts say the new development will serve as a catalyst for greater growth in the area, adding to downtown’s momentum.

“You couldn’t have asked for a greater adrenaline boost in an already strong market,” a director for Real Capital Analytics, Daniel Fasulo, said. “I really think that announcement is really just going to be the engine for future activity downtown.”

The building is likely to be both a blow and a boon for the developer of three towers yet to be built at the World Trade Center site, Larry Silverstein. Mr. Silverstein did not land the financial giant in one of his buildings, though the new tower is sure to inspire confidence in the site from the private sector and damp down fears that his buildings will flood the downtown market with an oversupply of office space. Mr. Silverstein owns towers 2, 3, and 4, while the Port Authority holds rights to the Freedom Tower and Tower 5, the site where JPMorgan will build.

Mr. Silverstein also owns 7 World Trade Center, which is currently about 70% leased, though a person familiar with negotiations says the developer is far along in talks to lease the top floors, accounting for virtually all of the remaining space.

Community members have expressed concern about the possibility of trading floors in the new tower cantilevering over an adjacent park, which they say would block sunlight and change the site plan.

“The master plan had the park designed in a certain way,” the chairwoman of the World Trade Center Redevelopment Committee at Community Board 1, Catherine McVay Hughes, said. “The big concern is, if the cantilevering is there, will there be shadows in the surrounding community on the park there, in the memorial and on the other residential and commercial buildings?”

Approval of the deal would need to come from the Port Authority board, and any modifications to the environmental impact statement for the site would need to be accepted by the state-run Lower Manhattan Development Corporation.

A spokesman for JPMorgan declined to comment.