PHOENIX — A free-market advocacy group contends the decision by Gov. Jan Brewer to expand the state’s Medicaid program will immediately increase the number of people in the program by nearly 90 percent.

The report released Monday by the National Center for Policy Analysis also says that the additional cost to Arizona taxpayers will hit $906 million in the coming decade, even with the federal government picking up most of the cost. And it says the fallout from the Affordable Care Act will be fewer people who are buying their own insurance.

Gubernatorial press aide Andrew Wilder said he could not comment on cost estimates because the report — essentially a four-page briefing — does not explain its assumptions.

But Wilder said the claim that the Arizona Health Care Cost Containment System, the state’s Medicaid program, will add nearly 1.1 million people next year, on top of the close to 1.3 million now enrolled, makes no sense. He said the number of people who will qualify is only about a third of what the report says.

Matthew Benson who represents the Arizona Hospital and Healthcare Association, which supports, expansion, agreed.

“They’re seemingly pulling a lot of these figures out of the air,’’ he said.

But Michael Bond, a senior finance lecturer at the University of Arizona, one of the authors of the report, said the numbers are in line with what has happened in the past when Medicaid programs have been expanded.

The report comes ahead of the first court hearing on whether the expansion is legal.

Lawmakers who voted against the plan contend the assessment being levied on hospitals to pay the state’s share is really a tax. That distinction is critical as a tax increase constitutionally requires a two-thirds vote of both the House and Senate, something the legislation did not get.

The governor disputes the characterization. And she contends that a simple majority of legislators can determine if a two-thirds vote is needed.

The lawsuit is a last-ditch attempt by foes to block Arizona’s participation in the federal Affordable Care Act before it takes effect Jan. 1.

It lifts Medicaid eligibility to an adjusted figure of 138 percent of the federal poverty level, about $26,951 a year for a family of three, from the current figure of $19,530.

The change also will mean Arizona will again start enrolling childless adults in the program, something it halted years ago to cut spending.

The hospitals themselves agreed to the levy based on projections from AHCCCS that every hospital chain in the state affected by the levy would actually make money.

AHCCCS estimates hospitals will pay $75 million for the first six months of 2014. But the agency says they will get back that much, plus $108 million over the same period.

Monday’s report deals not with the legalities but with Brewer’s contention that Medicaid expansion will be a financial benefit to the state and the hospitals.

Bond said that immediate increase of 1.1 million is not just people who now will qualify for government-provided care. He said about a third of those now have private insurance but will abandon that.

Wilder, however, said AHCCCS estimates there are just 57,000 Arizonans whose income is above the poverty level but below the new income cap. And he said the state estimates there are 240,000 childless adults in poverty who qualify but until now have been turned away.