An unusual dispute between the United Federation of Teachers and the elite KIPP charter chain spilled into public view Thursday after the union issued a press release accusing KIPP Academy Charter School in the Bronx of threatening to fire teachers if they did not vote to decertify the union.

The union’s claims, which were filed with the National Labor Relations Board this week and are disputed by KIPP, assert that school administrators encouraged staff members to sign a petition that would bar the UFT from representing its teachers.

The situation is uncommon because most charter schools in New York City aren’t unionized and are built partly on the premise that union rules are impediments to a sound education. But KIPP Academy is a “conversion” school, one of few district schools that morphed into charter schools.

The school’s 16-year-old status as a district-cum-charter school is likely at the heart of the dispute over whether its staff members are contractually tied to the UFT. Union officials say roughly 80 of its teachers and other staff members are covered by the city’s contract with the UFT — an idea that KIPP disputes.

“Except for collecting your dues from every paycheck, the union has not ever actively represented you,” Jim Manly, the superintendent of KIPP schools in New York City, wrote in a letter Thursday to staff across the city. The union’s NLRB complaint “and the aggressiveness of their press release is a preemptive effort by the UFT to block your individual ability to decide whether or not you want to be represented by the UFT.”

The latest disagreement over whether the UFT can enforce the city’s contract at KIPP Academy seems to have started boiling over this summer, when the union filed a grievance that alleges a laundry list of contractual violations.

KIPP’s Manly characterized the union’s grievance as a way of making “fundamental changes in the way we educate our students.” He added that staff members had previously tried to get the union decertified in 2010, but were blocked by the UFT.

KIPP co-founder David Levin emphasized the unusual nature of the UFT’s complaint. “For the past 22 years, KIPP Academy’s success has been the collaboration and effort among our educators, students, and parents,” he wrote in a statement. “In all that time, the UFT has never been involved in our school or raised any issues or concerns before now.”

In an interview, union officials said the grievance was filed over the summer for clear contract violations, and that KIPP’s attempt to coerce teachers into rejecting the union was directed in retaliation.

“Charter school employees, like other workers, have a right under federal law to organize and bargain collectively, rights that charter schools must respect,” UFT President Michael Mulgrew said in a statement.

The NLRB will investigate the UFT’s complaint, but in the meantime, you can read their allegations, and KIPP’s full response to its staff members.