An estimated 14 million people would lose insurance coverage in 2018 under the new Republican healthcare plan, according to a budget analysis.
The long-awaited Republican plan was assessed by the Congressional Budget Office (CBO), a nonpartisan group of budget analysts and economists.
The CBO said the added number uninsured would rise to 24 million by 2026.
The bill would reduce the federal deficits by $337bn (£275bn) over the 10-year period, the report found.
Those savings could help House Republicans sell the new legislation – known as the American Health Care Act (AHCA) – to some conservatives who remain sceptical about costs.
President Donald Trump has backed the plan, which would replace former President Barack Obama’s signature healthcare bill, the Affordable Care Act, known as Obamacare.
Other key findings in the report
The CBO, along with the Joint Committee on Taxation, also found that five million fewer people would be covered under Medicaid, which covers low-income people, by 2018.
An estimated 14 million fewer people would enroll in the Medicaid programme by 2026, it said.
And the report found that by 2026, an estimated 52 million people would be uninsured, compared with the 28 million who would not be covered that year under Obamacare.
Analysis: Anthony Zurcher, North America reporter
Now we see why the Trump administration spent the past week attempting to play down the importance of the CBO’s scoring of the American Healthcare Act. The numbers are in, and the top-line increase in uninsured Americans in 2018 – more than 14 million next year – is staggering.
Conservatives will herald the long-term budget savings derived from the legislation as well as the reduced taxes. But the pain from the cuts to coverage and subsidies will be more immediate and focused on the poor and the elderly.
Moderates, particularly those up for re-election in 2018 in states that had expanded Medicaid coverage, may very well cite the CBO number as reason to run for the exits.
The House bill was already under fire from the right for enshrining what they saw as a new “welfare entitlement”, so any attempts to lesson the blow of the changes will be difficult. The odds in favour of eventual passage are growing longer by the day.
The Republicans know they have to do something to fulfil their “repeal and replace” campaign promises, but finding a solution will take all the political skill the congressional leadership, and the Trump administration, can muster.
How have Republicans reacted to the CBO report?
House Speaker Paul Ryan highlighted the CBO’s conclusions on deficit reduction and decreased premiums. “I recognise and appreciate concerns about making sure people have access to coverage,” Mr Ryan said.
“[O]ur plan is not about forcing people to buy expensive, one-size-fits-all coverage. It is about giving people more choices and better access to a plan they want and can afford.”
Health and Human Services Secretary Tom Price said the administration “strenuously disagreed” with the report’s findings on the number of people who would lose coverage.
“Right now, current law, we’ve got individuals who have health coverage but no healthcare,” he said after the assessment was released.
Mr Price contended the new plan would cover more individuals at a lower cost.
How have Democrats reacted?
Democrats jumped on the figures in the CBO assessment. California Representative Adam Schiff called the numbers “appalling”.
“Now we know why Speaker Ryan rushed to pass his repeal bill; CBO says it kicks 24 million off their healthcare in next 10 years. Appalling,” Mr Schiff tweeted.
Virginia Representative Don Beyer called it a “disaster”.
Why is Trump proposing a new bill?
During his campaign, Mr Trump promised to scrap most elements of Obamacare.
The legislation is hugely unpopular among Republicans, who claim it imposes too many costs on business and is an unwarranted government intrusion into the affairs of businesses and individuals.
They say the AHCA will lower costs and argue that statistics showing it will lower coverage are misleading.
Democrats have accused Republicans of attacking the legislation simply in order to attack the credibility of Mr Obama and the Democrat party.