Fourteen million Americans would lose coverage next year under a Republican plan to dismantle Obamacare, according to a government agency tasked with performing cost-benefit analyses of proposals.

The report by the Congressional Budget Office (CBO), released on Monday, dealt a potential setback to President Donald Trump's first major legislative initiative.

The CBO projected that 52 million people would be uninsured by 2026 if the bill became law, compared with 28 million who would not have coverage that year if the law remained unchanged.

Two House of Representatives committees have approved the legislation unveiled by Republican leaders last week that would dismantle Obamacare.

But it faces opposition from not only Democrats, but also medical providers including doctors and hospitals and many conservatives.

READ MORE: Americans split over replacing Obamacare

The CBO said in its report that the Republican plan would save $337bn in government spending between 2017 and 2026.

The savings would come primarily in reduced spending for Medicaid, an assistance programme for low-income families, and an end to subsidised health insurance, two of the hallmarks of Obama's policy.

Those changes mean that, by 2018, 14 million more people would be without insurance than if existing legislation was allowed to remain in place. That figure grows to 21 million by 2020 and 24 million by 2026, the CBO reported.

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The Trump administration was quick to defend the proposal.

"We disagree strenuously with the report," Health and Human Services Secretary Tom Price said at a briefing.

He said the plan being considered offers greater choice and puts patients and doctors in charge of healthcare, and not the federal government.

Price said numbers would remain up because people would not voluntarily leave Medicaid. However, he also noted that there would be some increase in the number of those not covered because the new law would not require healthcare insurance, as Obamacare does.

He also noted that the report overlooks the fact that people will have a greater choice under the Republican plan.

"They are going to be able to buy a coverage policy that they want for themselves and their family," he said.