New York City Mayor Bill de Blasio's ambitious plan to expand public pre-kindergarten for all 4 year-olds depends in part on the participation of Jewish, Christian and Muslim schools under a proposal that would permit religious instruction and prayers during midday breaks.

But civil liberties groups are already objecting to the plan, which would take effect next fall, saying that a prayer break in a publicly funded classroom may violate the constitutional separation of church and state.

"It's kind of like waving a red flag in front of a bull," said Barry Lynn, executive director of Americans United for Separation of Church and State. "This seems to be asking for a lawsuit."

De Blasio's free, full-day pre-kindergarten plan began last year as his signature initiative, a way to address his goals of narrowing the wealth gap by saving parents the cost of private pre-K tuition, and jump-starting the academic performance of the city's youngsters. It started last fall with 53,000 children, and the goal is to boost that number this year to 70,000.

Some pre-K classes are in public schools but many are run by religious groups and other community-based organizations under contract to the city. The latest proposal is aimed at increasing the participation of private religious schools, particularly the Orthodox Jewish schools that serve a growing portion of the city's population. A 2012 study by the UJA Federation of New York found that 40 percent of Jews in the city identified themselves as Orthodox up from 33 percent in 2002.

De Blasio's administration announced last month that pre-K classes next year will be permitted to break in the middle of the day for "non-program" activities such as prayer. Additionally, schools will be invited to operate six days a week in order to meet the minimum number of classroom hours.

It is similar to an arrangement at pre-K classes currently operating out of Roman Catholic schools.

Rocio Fidalgo, a spokeswoman for the Roman Catholic Diocese of Brooklyn, said 22 of the diocese's schools are participating now and 14 more have applied for next year. An optional half hour of religious instruction takes place at 8 a.m., with the regular school day starting at 8:30 a.m.

But a prayer break in the middle of the day is seen by civil libertarians as more problematic than a prayer before the regular school day starts. The midday break means schools have to provide a space for any kids who "opt out" of the prayer and something for them to do.

Donna Lieberman, executive director of the New York Civil Liberties Union, called the break "an end run around the separation of church and state."

If your child was born in 2011, he or she is eligible to attend public pre-K classes in September 2015. The application period runs from March 16 to April 24. To apply, click here.