Investment analysts are keeping a close eye on a Long Island judge's decision to eliminate a controversial suburban tax that helps fund the Metropolitan Transportation Authority.

On Tuesday, Moody's Investor Service said that if the ruling to do away with the Payroll Mobility Tax isn't set aside, it would consider downgrading its A-2 rating and stable outlook for $16.6 billion in outstanding transportation revenue bonds scheduled to be sold next week.

"The timing and outcome of the appeals by MTA and the state are uncertain," Moody's said in a statement.

The tax, which forces employers in the MTA's region to hand over 34 cents of every $100 of payroll to the agency, was declared unconstitutional by Nassau County Supreme Court Justice Bruce Cozzens on Aug. 22. The MTA is appealing the ruling.

"If the appeal is lost, the MTA's TRB (transportation revenue bonds) credit would likely be placed on review for possible downgrade," Moody's said in the statement.

If the MTA is forced to make up for a $459 million hole in its budget caused by eliminating the tax, Metro-North fares could jump 32 percent, according to a study released Monday.

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Similarly, fares on the Long Island Rail Road could spike 46 percent, the study by the budget watchdog group Citizens Budget Commission found.

In addition to fare hikes, the dramatic loss in revenue likely would result in service cuts of 16 percent on the LIRR and 15 percent on Metro-North, according to the commission.

"The smiles of the suburbanites may turn to frowns even if the decision is sustained on appeal because the revenue lost from the PMT could be made up in a way that is far more burdensome to suburban commuters," researchers Charles Brecher and Raul Jain wrote in the commission's analysis.

The MTA has expressed confidence that Cozzens' ruling will be overturned on appeal because several other similar rulings already have suffered the same fate.

If the ruling stands, the price of a monthly ticket from White Plains to Grand Central Terminal could jump from $229 a month to $302, the commission predicted. And the price of a monthly ticket from Ronkonkoma, Long Island, to Penn Station would go from $334 to $488.

New York City employers, who contributed $987 million of the $1.4 billion the MTA collected through the tax last year, did not join their suburban neighbors in challenging the tax.

The tax was created in 2009 at a time when the MTA, which operates subways, buses and commuter railroads, was facing record deficits. A dozen counties from MTA's region, including Westchester and Putnam counties, sued to have the law overturned in 2010.

Republican lawmakers had cheered the judges' ruling.

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"We've won an important victory with the court's decision that this unfair burden on taxpayers was unconstitutional," Westchester County Executive Rob Astorino said when the decision was announced.