Three prominent local schools chiefs have joined forces to push the Maryland General Assembly to preserve funding for public education when it convenes in Annapolis for a special session next week.

Last month, the legislature ended its regular session without approving a package of tax increases that had been the subject of lengthy negotiations. The inaction raised the prospect of $500 million in funding cuts to education and other services taking effect July 1.

Prince George’s County Schools Superintendent William R. Hite Jr., Montgomery County Schools Superintendent Joshua P. Starr and Baltimore City Schools Chief Executive Officer Andres A. Alonso sent a letter Monday to Gov. Martin O’Malley (D) and legislative leaders, urging them to expand funding for education and expressing their “collective concern” about the budget impasse.

O’Malley announced Friday that he would call a special session to start May 14 at which legislators will take up the tax package.

Takirra Winfield, an O’Malley spokeswoman, said Monday the governor shares the concern of the schools chiefs.

“We agree that the legislature should pick up where they left off” when it adjourned, Winfield said. “If the ‘doomsday budget’ takes effect, our school systems face dire cuts.”

Under that scenario, public schools would lose $128.8 million in an annual supplemental education grant that benefits counties with higher costs of living and higher rates of poverty. The grant is known as the Geographic Cost of Education Index, or GCEI. Prince George’s, Montgomery and Baltimore collectively would lose $94 million.

The systems Hite, Starr and Alonso lead educate a third of Maryland’s students. The three systems also account for nearly two-thirds of the black and Hispanic students statewide and more than half of those who come from families poor enough to qualify for meal subsidies.

In the letter, the school leaders asked O’Malley, Senate President Thomas V. Mike Miller (D-Calvert) and House Speaker Michael E. Busch (D-Anne Arundel) to restore the GCEI and other formula funding for public education. They noted that the three school systems have received a majority of the geographic-index funding during the past five years and that the three have made significant gains in state testing.

“The elimination of the GCEI would be a huge setback in the gains our students have made,” the letter said. “If we are going to make good on our commitment to educating children equitably, regardless of their geographic location, we must maintain, and even increase, our investment in education.”

Hite and Starr said reductions would hurt the strides being made by the systems.

“We’ve been cutting for the last three years. That’s why we call this devastating,” Hite said. He said a $51 million cut in the Prince George’s schools’ $1.65 billion budget (counting the possible loss of funding from GCEI and other sources) could result in increased class sizes, furloughs and cuts in programs that affect college readiness.

Starr said it was unclear how a potential $41 million reduction in state funding, from a county schools budget that exceeds $2 billion, would affect services in Montgomery. But he estimated that the funding amounts to roughly 600 positions.

“We’re already at bare bones,” Starr said. “It would just be one more hit to us.”

Staff writer Michael Alison Chandler contributed to this report.