A state judge has struck down a plan by the Bloomberg administration to rent sports fields on Randalls Island to private schools, saying the administration had improperly excluded the City Council from reviewing the proposal.

It is not clear how much resistance, if any, the plan will meet at the council. Under the proposal, 20 private schools would receive exclusive rights to most of the sports fields on the island between 3 p.m. and 6 p.m. on school days for the next 20 years. In return, the schools, which include Brearley, Collegiate, and Dalton, would pay more than $52 million to the city, to be used to help defray the costs of building new sports fields on the island.

The plan has come under criticism because other children from nearby public and parochial schools would be excluded from playing on the fields during those hours. The plan did receive approval from the city’s Franchise Concession and Review Committee, most of whose members were installed by Mayor Bloomberg. It had not, however, gone through a separate land use review process that would have required the council’s review. In a rather technical opinion, a judge, Shirley Kornreich, said yesterday that the Bloomberg administration should have steered the plan through the review process that required council approval.

The decision annuls the agreement and ensures that one council member, Melissa Mark Viverito, will now play a major role in deciding whether the plan ultimately succeeds or fails.

Ms. Viverito’s district includes Randalls Island, as well as surrounding East Harlem and parts of the Bronx, where many of those who play on the fields live. Other council members will likely follow Ms. Viverito’s lead in supporting or opposing the plan.

In an interview, the council member praised the ruling and said, “We believe there have to be greater concessions and more access to the fields” for the public. She criticized the price the schools would pay each year — $2.63 million — as too little.

“For me, to warrant that much control of the fields, that wasn’t the price,” she said.

When asked what sort of a an agreement she would support, Ms. Viverito said, “I can’t give you an answer as to exactly what that it is, because we haven’t had that opportunity to have that kind of a public review.”

In one sense, the Bloomberg administration’s plan would codify an arrangement already in place. Private schools already get permits for exclusive use during after-school hours for most of the 36 fields currently on the island. The plan would cover most of the costs to build sports fields on 12.5 acres of land that is either vacant or currently used for parking.

The plan has faced opposition from parks groups, Manhattan’s president, Scott Stringer, and residents of East Harlem and the South Bronx who use the fields. A spokeswoman for the city law department said the city was “reviewing the decision.”