NEW YORK, March 20 (Reuters) - Five New York City bar associations will appeal last week's ruling allowing the city to proceed with a plan to shift tens of thousands of cases involving indigent defendants from private lawyers to the Legal Aid Society and other groups.

The 3-2 decision, issued March 15 by the Appellate Division, First Department, held that the city's proposal did not require the bar groups' consent under Article 18-B of the County Law, which governs the assignment of counsel to indigent defendants.

The disputed plan would only affect cases in which the initial legal aid group assigned to a case cannot provide representation due to a conflict.

The bar groups, which were entitled to an appeal of right, filed a notice of appeal Monday.

"The County Bars' statutory role in helping to fashion and administer New York City's indigent defense system -- a role they have dutifully served for nearly half a century to ensure that the rights of New York's indigent defendants are protected -- is simply too important to permit the City to move ahead with its plans to impose a new indigent defense system in violation of state law," said Jonathan Pressment, lead attorney for the groups, in a statement Tuesday.

Starting in the 1960s, the Legal Aid Society and other legal aid organizations were initially assigned to represent indigent defendants in all criminal cases.

Until 2010, cases in which aid groups had a conflict were reassigned to approximately 1,100 private attorneys, known as 18-B lawyers, who were identified by the county bars. But the city decided to solicit contracts from aid groups to handle conflict cases instead of continuing to rely solely on 18-B lawyers.

Shortly after, the New York County Lawyers' Association and bar associations from the city's other four boroughs filed a lawsuit against the city, claiming their consent was required to amend the procedure of assigning conflict cases, since they helped design the original 18-B plan.

Legal Aid participated as an intervenor on the city's side, while the New York Criminal Bar Association was an intervenor on the bar groups' side.

Under the city's new plan, 18-B lawyers would continue to handle all homicide cases, to ensure that those defendants receive the expertise of veteran attorneys.

John Feinblatt, criminal justice coordinator of New York City, said in a statement, "Two courts have rejected the petitioner's arguments, and we look forward to resolving the issue once and for all in the Court of Appeals."

The case is In re The New York County Lawyers' Association, et al., v. Bloomberg et al., Appellate Division, First Department, No. 5389.

For the bar associations: Jonathan Pressment, David Siegal and Kendyl Hanks of Haynes and Boone

For the New York Criminal Bar Association: Zoe Jasper and Alun Griffiths of Satterlee Stephens Burke & Burke

For the city: Julian Kalkstein, Larry Sonnenshein, Thaddeus Hackworth and Jonathan Pines of the New York City Law Department

For the Legal Aid Society: Daniel Kolb, Daniel O'Neill and Jennifer Marcovitz of Davis Polk & Wardwell

(Reporting by Joseph Ax)

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