More than 20 Detroit Public Schools will be closed Monday, the district said, after the teachers union urged members to call in sick in a protest over funding issues.

The move by the Detroit Federation of Teachers was announced at a press conference Sunday. It comes a day after the district's transition manager said DPS will have no money to continue paying teachers this summer without further funding from the state.

In March, Gov. Rick Snyder signed into law emergency funding that is keeping the district operating through the end of the school year as the state Legislature considers a $720 million restructuring plan that would pay off the district's enormous debt.

Former bankruptcy Judge Steven Rhodes, who was appointed this year to oversee the district, also said Saturday that DPS will be unable to fund summer school or special education programs after June 30.

"The district's promises are no longer worth the paper that they are printed on," Terrence Martin, executive vice president of the DFT, said at the news conference, according to the Detroit Free Press.

"While we recognize that this puts Detroit's parents and communities in a difficult situation, the district's broken promises and gross negligence leave us no choice," he said.

Teacher strikes are illegal under Michigan law. Sick-outs earlier this year shut down schools, causing tens of thousands of students to miss class.

District spokeswoman Michelle Zdrodowski said in a statement late Sunday that 21 schools would be closed Monday.