Pittsburgh Public Schools said late Friday that the union representing their teachers and other employees has notified members it will seek authorization for a strike.

Pittsburgh Federation of Teachers president Nina Esposito-Visgitis confirmed that the union will mail authorization ballots to members.

“We do not take this lightly,” Ms. Esposito-Visgitis said in a phone interview Friday night.

The news comes as union members — 2,400 teachers, 565 paraprofessionals and 20 technical-clerical employees — continue to work under the terms of a contract that expired June 30.

If union members vote for authorization, it does not necessarily mean a strike will occur. According to the district, the union would have to give 48 hours of notice before striking.

Ms. Esposito-Visgitis said she could not say how likely it was that the union would strike.

“We have been negotiating in good faith for a year and a half,” Ms. Esposito-Visgitis said. “We are willing to continue negotiations.”

In a press release, the district said it also wanted to continue negotiations.

“We continue to work diligently with PFT leadership toward a resolution that reflects our deep respect for our outstanding teachers while also prioritizing our mission of providing the highest quality education possible for every student,” the district said.

Pittsburgh teachers haven’t gone on strike in more than 40 years — the last one lasted for eight weeks, from December 1975 to January 1976.

The Pittsburgh Federation of Teachers last voted to authorize a strike in October 2007. A walkout was averted when the union and district agreed on a contract.

First Published: January 27, 2018, 3:52 a.m.