An estimated 14 million more people in the United States would lose insurance coverage in 2018 under the new Republican healthcare plan, according to a budget analysis office.
The Congressional Budget Office (CBO), a group of budget analysts and economists, released its assessment on the long-awaited Republican bill.
The group said the number of uninsured would jump to 24 million by 2026.
The plan to replace the Affordable Care Act has faced backlash from Democrats and even some Republicans, as well as medical providers, doctors and hospitals.
The nonpartisan estimate would probably be used by Republicans and Democrats in the debate over a finalised version of the legislation.
Republicans have said the goal of the plan is to lower costs and that coverage statistics are misleading due to the high out-of-pocket costs under President Barack Obama's signature health law.
Democrats jumped on the figures in the new assessment.
California Representative Adam Schiff tweeted that the numbers in it were "appalling" while Virginia Representative Don Beyer called it a "disaster".
"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.