The union representing New York City firefighters plans to file a lawsuit Tuesday morning against the speaker of the City Council, alleging an "agenda shrouded in secrecy" when it comes to FDNY disability pension benefits.

The court papers will charge Speaker Melissa Mark-Viverito's office stonewalled efforts to hold a public hearing on the benefits and refused to release documents under the Freedom of Information Law, a spokeswoman for Uniformed Firefighters Association said Monday.

The FDNY union has been lobbying for higher disability pensions for their newest members. A spokesman for Mark-Viverito said her office hadn't seen the complaint but the City Council operates in an "open and transparent fashion."

The union would not comment Monday night on details of the suit or damages sought. The court documents are expected to be shared at a news conference Tuesday at New York State Supreme Court House in Manhattan.

Before a 2009 state law change, firefighters, police officers and other uniformed city workers received 75 percent of their last year's salary if injured on duty. Recent hires hurt on the job receive half pay, with the benefits further cut by Social Security disability benefits.

Mayor Bill de Blasio and other elected officials have said the city's fiscal health would be threatened by full benefit restoration and a middle ground must be sought.

With little advance notice, City Council members last month pushed through approval of new pension package in line with de Blasio's proposal for benefits closer to those in place before 2009 but not to the extent sought by the FDNY and NYPD unions. The city legislation has no effect without approval by Albany lawmakers.