Subway operators and conductors may refuse to work because they lack up-to-date respirator masks they’re required to wear in emergencies, a top Transport Workers Union official said Saturday.

The MTA failed to replace masks that had a use-by date at the end of May, said Kia Phua, the union’s vice president of rapid transit operations.

Train operators and conductors are required to don the masks if there’s an emergency that requires them to lead riders to safety on subway tracks. Without the masks, workers and riders are in increased danger, Phua said.

“It’s a safety issue, and they haven’t given us what we need,” said the union official.

Unless the masks are replaced, about 75 percent of conductors and operators might refuse to operate trains, Phua said. “It’s a possibility,” he said. “If this happens, there will be a very limited amount of service.”

Phua said he’s been told the masks won’t be replaced until late this month.

Phua gave The Post copies of his communications with MTA brass about the issue.

“The operation and Instructions manual for the escape masks clearly states, ‘Never use….if respirator expiration date is exceeded,’ ” he wrote in an email to several top MTA officials and Gov. Cuomo.

“There are some cases where there are Train Operators and Conductors that DO NOT have an escape mask AT ALL! ,” the email said. “This to me poses a VERY SERIOUS SAFETY ISSUE where God forbid a more serious event were to occur on the train, our train crews will not be able to lead the customers to safety.”

Phua said he sent the email on Thursday after having no luck with other MTA managers and got no response.

Subway riders have endured alarming safety incidents in recent weeks caused by train, track and signal failures. They include an A train derailment in Harlem two weeks ago, and at least three incidents where riders felt so trapped that they walked the tracks to get off their trains.

A government source with knowledge of the situation said the company that provides the masks has sold much of its stock to the US military and humanitarian groups working in Syria, including Doctors Without Borders. Because of those sales, the source said, the company can’t immediately fill the MTA’s order for replacements.

MTA officials said they’re working on the problem. “The safety of our employees and customers is our top priority and we will begin to take delivery of new masks by the end of the month,” said MTA spokesman Kevin Ortiz.

Agency officials did not respond to questions about what they would do if the workers refuse to run the trains.