A strike by New York City school bus drivers that had been threatened for weeks will start Wednesday morning, affecting 152,000 students, the president of the union representing the drivers announced Monday.

Michael Cordiello of Local 1181 of the Amalgamated Transit Union said more than 8,000 bus drivers and matrons will participate in the strike, brought about by a dispute over job protections in any new bus company contracts for the bus routes. Matrons accompany the children on the bus and make sure they get on and off the bus safely.

"With its regrettable decision to strike, the union is abandoning 152,000 students and their families who rely on school bus service each day," Mayor Michael Bloomberg said in a statement. "As Chancellor (Dennis) Walcott and I have said, the City will take all steps available to ensure that those who are impacted have the support they need, and we are now activating the protocols we put in place in the event of a strike."

Under the city's strike contingency plans, students would receive free MetroCards for mass transit. Parents or guardians of younger children also would get the cards.

Families of special needs students would be reimbursed for private transportation. Of the 152,000 students who use the buses, 54,000 are disabled and would face extra hardships in trying to find alternative transportation.

There are 1.1 million students in the New York City schools. While the majority don't use school buses, those that do are among the youngest ones.

The city wants to cut transportation costs and has put bus contracts with private bus companies up for bid. The union is decrying the lack of employee protections, saying current drivers could suddenly lose their jobs once their contracts are up in June if the companies they work for aren't the ones getting the new contracts and the new contract holders don't hire them.

The mayor said the privately-contracted drivers were demanding job security, or Employee Protection Provisions.

However, such a guarantee is not allowed in contracts between drivers and companies chosen by the city to provide bus services, the city says. The state Court of Appeals in 2011 barred the city from including EPP because of competitive bidding laws. Hence, the mayor said, the city cannot accept the union demand for an EPP clause.

"Let me be clear: the union's decision to strike has nothing to do with safety and everything to do with job protections that the City legally cannot include in its bus contracts," Bloomberg's statement said. "We hope that the union will reconsider its irresponsible and misguided decision to jeopardize our students' education."

The drivers' contracts expire on June 30, and Bloomberg said the city must "go ahead" in seeking competitive bids that would save money.

Cordiello refuted the idea that the EPP was not allowed. He said the 2011 Court of Appeals ruling was based on the fact that the city at the time did not offer the judges enough evidence to support its contention that the EPP job-security clause does not push up costs.

The union president also challenged the mayor's assertion that both sides in the labor dispute had been talking "all along."

Not true, Cordiello said. He said he'd met the chancellor for only 20 minutes last week, and the deputy mayor for an hour.

The head of another union, Teamsters Local 854, said its members would not go on strike with the bus drivers because their contracts don't allow it, but they would not cross any picket lines. Local 854 represents drivers, matrons and mechanics, some of whom work alongside members of ATU.

Dan Gatto, president of the local, put the blame on Bloomberg. "For weeks now, City Hall has refused to discuss the job-killing provisions they are insisting on as part of new contracts with bus contractors," he said.

"We urge Mayor Bloomberg and his administration to work with the ATU to resolve this dispute before a job action is required," Gatto said.

In 2011, the city said the union was threatening to strike over bus route contract bids, but the union said the warning was a false alarm. No strike took place.