CHICAGO (Reuters) - Illinois state workers cannot be paid without an enacted budget for the fiscal year that begins Wednesday, State Attorney General Lisa Madigan said on Monday as a major labor union warned it could sue over any missed paychecks.

The Democrat-controlled legislature and Republican Governor Bruce Rauner have been at an impasse over a spending plan. Madigan said "the Illinois Constitution and statutes prevent the comptroller from continuing to pay expenditures, including the state’s payroll, without a budget, and even a court cannot order all of these payments to be made."

Roberta Lynch, executive director of the American Federation of State, County and Municipal Employees Council 31, the state's biggest labor union, said workers will remain on the job and that the union will be prepared to take legal action to ensure they are paid on time and in full.

"We urge the governor to stop demanding that the General Assembly approve his unrelated agenda items that would harm the middle class as a precondition to budget talks, and instead work with lawmakers to fairly fund state government and the important services it provides,” Lynch said.

The first fiscal 2016 payroll date is July 15.

Rauner sent state workers a letter pledging that he will "do everything within my power to ensure you don't miss a single payroll," according to political blog Capitol Fax.

Catherine Kelly, Rauner's spokeswoman, said the governor was looking for an agreement with unions that workers must be paid to remain in compliance with state and federal labor laws.

"We are ready to reach a similar agreement with state workers and hope the Attorney General reconsiders her efforts to block state workers from getting paid," Kelly said, referring to previous agreement made in 2007. 

Rauner last week vetoed most of the $36.3 billion budget passed by Democrats. The governor has said he will not consider new revenue for the budget unless the legislature adopts his so-called turnaround agenda that includes legislative term limits, a local property tax freeze and workers' compensation reforms.

The Illinois House has scheduled hearings for Tuesday and Wednesday on the potential impact of a state government shutdown, inviting the heads of 15 state agencies to testify.

(Reporting By Karen Pierog; Editing by Diane Craft)