Some striking city school bus drivers may start losing benefits for taking part in the two-week work stoppage.

Neil Strahl, president of the Staten Island-based school bus company Pioneer Transportation Corp., sent a letter to his drivers and matrons warning that if they don't return to work by Friday morning, they'd lose their health benefits.

Carolyn Daly, a spokeswoman for Pioneer, said the letter was not issued as a threat, but to tell members of Local 1181 of the Amalgamated Transit Union what they would lose by not operating their routes.

"If they're not working, they're not entitled to benefits or pay. That's the downside of the strike," she said.

Daly said if drivers come back to work after Friday's deadline, they would get back their benefits and pay. She would not say how many drivers and matrons are employed by Pioneer.

The union declined to comment, and showed no signs that its 8,000 members would end the strike anytime soon.

The union wants the mayor to put employment protection provisions in bids for 1,000 new school bus contracts. The provision would guarantee jobs for drivers, based on seniority, at new companies.

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Mayor Michael Bloomberg has refused to include the protections because courts deemed them illegal.

He also refused to meet with the union during negotiations, contending their feud is with the bus companies, not the city.

Daly disagreed with the mayor about this, noting that it was Bloomberg who called for new contracts.

"There are issues with the city that we can't resolve. But we can get their paychecks back," she said.