NEW YORK, Aug 9 (Reuters) - New York City's labor agreements with 50 unions do not violate federal labor law, a Manhattan judge has ruled, a decision the city says will clear the way to finish $6 billion of infrastructure projects.

Last year, two construction trade groups, the Building Industry Electrical Contractors Association and United Electrical Contractors Association, challenged the accords.

Instead of acting to secure the best deal for the taxpayers, the city was using the pacts to write its own labor rules for the projects and handpick favored unions and contractors for the vast majority of the work, the suit said.

But in a ruling handed down Thursday, U.S. Southern District Judge Robert Patterson sided with the city.

"Plaintiffs have failed to show that the city's decision to engage in these project labor agreements with Building and Construction Trades Council was motivated by anything other than its interest in managing its construction projects in an efficient manner at a manageable cost," Patterson wrote.

Signed in 2009 by the city and the Council -- which represents about 50 construction unions, the deals cover a range of public infrastructure projects, including a new police academy and a larger 911 call center.

'CITY MOTIVATED BY EFFICIENCY'

City officials estimate that the agreements, which run through 2014, will save $300 million over the next three years.

In addition to laying out uniform work rules, the pacts require most contractors to get at least 88 percent of their labor through the construction trades council union referral systems, incorporate the affiliate unions' collective bargaining agreements and include a broad no-strike clause.

Patterson in his decision wrote: "The terms of the (accords) lead ineluctably to the conclusion that the city was motivated by efficient, cost-effective and timely completion of their work, and plaintiffs point to no credible alternative purpose underlying the city's actions."

The ruling will allow major public works projects to be built and "ensure that future construction projects will be completed in a cost-effective and efficient manner," a city attorney, Jonathan Becker, said in a statement.

Alan Pollack, who represented the plaintiffs, said his clients will appeal. The ruling will hurt city taxpayers by driving up construction costs, he said.

The case is The Building Industry Electrical Contractors Association et al v. City of New York, in the U.S. District Court for the Southern District of New York, No. 10-8002.

For the BIECA and UECA: Alan Pollack, Felicia Ennis and Jonathan Rich of Robinson Brog Leinwand Greene Genovese & Gluck.

For the city: Jonathan Becker, Eric Jewell and Steven Cushman of the New York City Law Department.

For BCTC: Carol Pennington of Colleran O'Hara & Mills.

(Reporting by Jessica Dye)