The United Federation of Teachers filed suit in state Supreme Court on Tuesday to force the Department of Education to hand over copies of official e-mails that it has been requesting since May 2010.

The union wants to see e-mails between various city officials and education groups regarding proposals to phase-out failing schools and to open new charter schools. The suit accuses the city of violating the state’s Freedom of Information Law by repeatedly saying it needs more time, calling that a “constructive denial” of the requests for information.

The union’s president, Michael Mulgrew, issued a statement noting that Mayor Michael R. Bloomberg “rushed” at the chance to release teacher ratings to the media with the names of individual teachers.

“His lawyers in that case claimed that the people’s right to know is ‘basic to our society,’” Mr. Mulgrew said. “But when it comes to a simple request about City Hall’s correspondence with charter school proponents, it’s delay after delay after delay. You’d think City Hall had something to hide.”

A spokeswoman for the city’s Law Department said the agency had not yet been served with any papers, and therefore couldn’t comment on the suit.

The union and the N.A.A.C.P. won a lawsuit in 2010 when the city tried to close 19 struggling schools. The city tried again in 2011 and succeeded in phasing out a total of 22 schools. The union, however, has never given up that battle.

According to court papers in the lawsuit over the e-mails, the union sent a request to the city on May 28, 2010, seeking copies of e-mails between and among Department of Education higher-ups, including then-Chancellor Joel I. Klein, several deputy chancellors, Deputy Mayor Howard Wolfson, the C.E.O. of the New York City Charter School Center, the leadership of Democrats for Education Reform, and political consultant Bradley Tusk.

In its response a month later, the Education Department argued that the request was not “reasonably described” with sufficient details to locate and identify the records the union wanted. The city then continued to report back each month that it needed more time to deal with the “expansive nature” of the union’s requests.

In a letter on March 22, the Education Department’s general counsel, Michael Best, told the union that a search of Mr. Klein’s and Deputy Chancellor Kathleen Grimm’s e-mails alone “has resulted in approximately 10,000 potentially responsive e-mails.”