But a lawsuit filed on Tuesday against the museum complains that most museum-goers have no idea that the "recommended" $US25 ($A24.40) entrance fee is nothing more than a suggestion.

The plaintiffs include a member of the museum, along with two Czech tourists who purchased single-day admissions. They argue the museum employs misleading signs and other techniques to dupe its 6 million annual visitors into believing they must pay to gain access.

The museum's rent-free lease with the city mandates that it open its doors to the public for free on multiple days a week, although it is permitted to ask for a voluntary fee. But the lawsuit says the museum deliberately deceives its visitors into believing that the charge is mandatory.

Signs above the admissions desk that list the entrance fees feature the word "recommended" in small type below the word "admissions" in larger, bold type. The lawsuit also pointed out that visitors are funneled in lines to the admissions desks, where cashiers await to collect the fee.

"MMA has misled, and regularly misleads, members of the general public to believe, on all days of the week during times when the MMA is open, that they are required to pay the Admission Fees in order to enter Museum Exhibition Halls," the lawsuit claimed.

Museum spokesman Harold Holzer said in an email that the museum is "confident that our longstanding pay-what-you-wish admissions policy meets the spirit and letter of our agreement with the city ... and ensures that the Met is fully accessible to and affordable by all."

The law firm that filed the case, Weiss & Hiller, previously filed a similar lawsuit on behalf of two museum members seeking an injunction requiring the museum to make its policy clearer to visitors. The suit is still pending.

Tuesday's complaint asks for an injunction as well as unspecified damages for all museum visitors who, like the three named plaintiffs, paid to enter with a credit card.

"There's absolutely no doubt in my mind that, as reflected in the complaint filed today, No. 1, an overwhelming majority of people who visit the museum are completely fooled into believing that they are required to pay the museum's admission fees; and No. 2, museum officials know all about it," said Michael Hiller, the lawyer for the plaintiffs.

The museum, known colloquially as "The Met," is one of the world's largest and most acclaimed art museums.

Reuters