In a step toward opening up New York’s real estate marketplace, the Real Estate Board of New York announced yesterday that it plans to launch a public Internet portal containing all the exclusive apartment sales and rental listings of its 319 members, including the city’s biggest brokerages.

While similar systems exist in many real estate markets around the country, establishing a comprehensive, shared, and free listing system on the Web has been a long, hard slog in New York City. Earlier attempts at cooperation among the city’s largest and most competitive real estate firms crashed and burned amid squabbling.

Currently, brokers with member firms must share information with one another about their exclusive listings within 72 hours, and about 10,000 to 15,000 sales and rental listings are available to brokers from member firms. The new portal, to be released early next year with the fanfare of a seven-figure advertising budget, would offer these listings online to the public.

Potential buyers can now find most apartment listings online, scattered on a variety of Web sites that belong to the firms, or in paid-for classifieds. In the proposed Web portal, the most comprehensive list available would be on one site. When a potential buyer clicks a listing, the buyer would be linked to the listing firm’s Web site.

The president of the Real Estate Board of New York, Steven Spinola, said the system would make apartment shopping easier for potential buyers and give another marketing tool to brokers.

About 75% of buyers now consult the Internet before buying an apartment, according to real estate experts. Some national real estate search engines have appeared online recently, but they contain only a fraction of the New York City listings that are controlled by members of the Real Estate Board. Some of the city’s largest firms, such as the Corcoran Group and Prudential Douglas Elliman, have been reluctant to share all of their exclusive listings with the search engines, like Trulia or Street Easy.

Some real estate experts say that it is not immediately clear what large real estate firms would gain by sharing their listings alongside the listings of smaller firms. An appraiser for Miller Samuel, Jonathan Miller, said the new site would likely favor smaller real estate firms who would increase their exposure by piggybacking the big firms.

Mr. Miller said that a shared public database could erode some brand distinction between companies that spend a lot of money on promoting brands. Traditionally, New York’s firms have been associated with certain neighborhoods, kinds of apartments, or income levels, and that is changing, he said.

“REBNY seems to be in a position now of blurring the distinctions among their members,” Mr. Miller said.

Mr. Miller also said that it would be difficult for the Real Estate Board to compete with the technological savvy of the Silicon Valley-funded Web sites, like Trulia.

The president of Prudential Douglas Elliman, Dottie Herman, said the shared public portal was still in the very early stages of discussion.

“This thing with REBNY is not a done deal yet. Nobody has really agreed 100% to anything yet,” Ms. Herman said. “There are some roadblocks that people haven’t agreed to at this point.”

One point of contention, Ms. Herman said, was the amount of information that would be available on the shared site, and what information would be reserved for buyers who clicked on the listing broker’s home Web site. Another issue, she said, was the cost of promoting, running, and maintaining a shared site. Many members already sink millions of dollars into their own Web sites, and right now, the same information is available for the consumer, scattered on several different Web sites.

The president of the Corcoran Group, Pamela Liebman, said she fully supports the shared portal. She said that times have changed in New York real estate since the last failed effort.

“We are in a different place now. Everybody is sharing,” Ms. Liebman said. “It is a convenience tool for the public, but we will still totally control how we display our data and our images. We are not afraid of losing traffic to our Web site.”

The president of Warburg Realty Partnership, Frederick Peters, said that new portal would not drastically affect the city’s real estate landscape. He said the major change came about a decade ago when firms first started sharing information about listings with each other. Previously, a broker with an exclusive apartment listing was reluctant to split commissions with another broker who found a buyer.

Mr. Spinola said that it would be a major tool for firms of all sizes. He said that the site would help large firms by allowing them to market their listings less expensively than other potential marketing avenues. Several real estate sources said that if the Real Estate Board of New York’s portal took off, it could potentially harm revenues of the New York Times’s online real estate classifieds.

Mr. Miller, the appraiser, said that consumers would be more informed, but more information would be unlikely to affect prices or reduce the need for real estate brokers. “Brokers are still needed, but the clients or customers they are picking up are more informed,” he said.

The Web address of the new portal has not yet been determined, according to the Real Estate Board.