City nightlife industry insiders fear that the days of 4 a.m. last calls are numbered.

In most parts of Manhattan, bar and club owners say, it has become nearly impossible to open new nightlife establishments that are permitted to serve alcohol until 4 a.m. — the Prohibition-era curfew that is seen as a bedrock of New York City’s party town identity.

Community boards that now hold substantial clout with the New York State Liquor Authority are increasingly requesting that liquor licenses be tied to earlier closing times, often at or before 2 a.m., a number of advocates for the nightlife industry said.

“It is a trend that has certainly increased in the last six months, and if it keeps increasing there are going to be serious problems for the nightlife industry and the city’s economy,” a lawyer for the New York Nightlife Association, Robert Bookman, said. An independent study conducted at the request of his association found that city nightlife establishments garner 58% of their revenues between the hours of 1 a.m. and 4 a.m., he said. Under state law, which says the closing time for bars in the city is 4 a.m., the liquor authority lacks the power to place stipulations on the operating hours of bars and clubs. However, if a bar owner agrees to limit its hours of operation with the local community board, the liquor authority will write it into the license.

Community boards for years have provided the liquor authority with recommendations for liquor licenses, but only recently has their influence become a determining factor in designating them. “The new leadership that took over during the last few years of the Pataki administration has been dramatically more responsive to the community than any I’ve ever seen with many years of experience with the SLA, and I think that’s a good thing,” a state assemblyman who represents the Chelsea area of Manhattan, Richard Gottfried, said.

A spokesman for the liquor authority, William Crowley, said the agency determines liquor licenses on a case-by-case basis and follows the letter of the law.

Some say community boards are wielding their newfound power with the liquor authority to force bars to close earlier than ever before.

“More and more of the community boards are insisting that bars close at 2 a.m. or earlier,” Ben Leventhal, the editor in chief of Eater.com, a Web log that chronicles many of the liquor license issues arising in the city, said. “It’s become the community boards’ de facto bargaining chip.”

Matthew Piacentini, an entrepreneur poised to open a European-style parlor, said he decided to back out of plans to open the lounge in a commercial building on Hudson Street in TriBeCa after the community board stipulated he would have to close at midnight on weekdays and 1 a.m. on weekends. Mr. Piacentini pitched his establishment as an upscale anecdote to nightclubs for a clientele interested in having conversation. “If I could only be open for six hours there was no chance I could bring in the necessary revenue,” he said.

Brad Hoylman, the chairman of Community Board 2, which covers Greenwich Village and SoHo, said the board approves 90% of all liquor license applications, and 80% of those are tied to certain stipulations such as time constraints.

“In our neighborhoods, most people don’t want to live next to a nightlife establishment, he said. “What we try to do is be reasonable and have a fair negotiation with the applicant.”

Community Board 3, which covers the bar-saturated neighborhoods of the East Village and the Lower East Side, is widely described as the stingiest board in the city when it comes to obtaining a new liquor license. The board’s district manager, Susan Stetzer, said her board doesn’t stipulate closing times, but that the applicants themselves come to the board with their own closing times.

According to minutes from the board’s monthly meetings in November, December, and January, the most recent records available, not a single liquor license recommendation was granted to a bar that would close after 3 a.m. on weekends and 2 a.m. on weekdays.