JUNEAU — The state could face irreparable fiscal injury if Gov. Bill Walker’s plan to expand Medicaid coverage in Alaska takes effect next week, according to a lawsuit brought by state lawmakers challenging Walker’s authority to expand the program on his own.

In a court filing, attorneys for the Alaska Legislative Council say each dollar diverted toward implementing Medicaid coverage not authorized by state law is a dollar that instead could go to a legislatively authorized program.

They also say that if Walker’s plan takes effect Sept. 1 as planned and the state begins enrolling people under an expanded Medicaid program, it would not only violate the law but threaten to “engender massive confusion” and give some Alaskans “the erroneous impression that they are covered by Medicaid when that is not, in fact, so.” It also could create an administrative nightmare if expanded Medicaid begins and is later determined unlawful, the attorneys say in arguing that patients and providers stand to suffer irreparable injury.

The lawsuit, filed in Anchorage Superior Court on Monday, asks a judge to bar the administration from expanding Medicaid unless lawmakers approve doing so.

The state Department of Law argues that Walker has the authority to move ahead as he has, department spokeswoman Cori Mills said by email. The department will respond to the litigation in accordance with whatever schedule is set by the court, Mills said.

Walker told reporters Tuesday that the Anchorage-based law firm Dillon & Findley offered to help the department in the case as pro bono counsel. He said he accepted what he thought might be an unprecedented offer of services that will help significantly with the court costs.

The litigation centers on whether the population targeted by the expansion — people between the ages of 19 and 64 who are not caring for dependent children, not disabled and not pregnant, and who earn up to 138 percent of the federal poverty level — is a mandatory group for health care coverage under Medicaid or an optional group.

The federal health care law expanded eligibility for Medicaid, but the U.S. Supreme Court in 2012 said states could not be penalized for not participating in expanding the program.

The lawsuit contends the expansion population is an optional group that cannot be covered unless such coverage is approved by the Legislature.

The Legislative Council, made up of House and Senate lawmakers, voted 10-1 last week to sue Walker over his plan, with members calling it as a separation of powers issue. That point is raised in the litigation.

The lawsuit also says the potential use of funds from the Alaska Mental Health Trust Authority to cover the administrative costs of expansion is problematic and that Walker’s plans violate language that legislators added to the budget seeking to bar expansion.

The trust’s board is scheduled to consider this week a proposal to allocate about $1.6 million to help with administrative costs related to expansion. The Legislature’s top attorney and state attorney general have said the budget language could be unconstitutional.

Walker last month announced plans to accept federal funds to expand Medicaid after lawmakers previously tabled his expansion legislation for further review. He followed a process, spelled out in state law, by which a governor can go through the Legislative Budget and Audit Committee when seeking to spend more in federal or other funds on a budget item than allocated by the Legislature.

Walker has said he stands firm on his decision to move ahead with expansion.