Nassau County will challenge an appellate court's decision upholding the MTA's payroll mobility tax, a county spokesman said.
The Appellate Division on Wednesday overturned a lower-court ruling in favor of Nassau, which sued the Metropolitan Transportation Authority in 2010 seeking to abolish the tax.
"We will appeal the ruling," said Brian Nevin, spokesman for County Executive Edward Mangano.
The county has argued that the tax violated the state constitution because it changed the tax policies of individual municipalities for a purpose that did not benefit the entire state.
But in their decision, the appellate court ruled that the tax, which charges large employers in the MTA region 34 cents for every $100 of payroll, "serves a substantial state concern" because it funds transportation services throughout the entire MTA service area.
A challenge of the decision would have to be made to the Court of Appeals, the state's highest court.
The tax was created in 2009 as part of a state bailout of the MTA as it faced a nearly $2 billion budget gap. It generates about $1.2 billion in annual revenue for the MTA.
William Schoolman, a Suffolk business owner who filed the original lawsuit challenging the constitutionality of the tax, said that the MTA, as a public authority, should be "self-sustaining."
Schoolman, of Bohemia, who owns Classic Coach Cos., said he has dropped his suit against the MTA.
Other suburban municipalities have unsuccessfully challenged the tax, saying it doesn't benefit many of their residents.
But Ryan Lynch, associate director of the nonprofit Tri-State Transportation Campaign, said that even if Long Island business owners or their employees don't use the MTA, they benefit from the higher property values and reduced traffic congestion that it creates.