The Senate Republicans' healthcare plan would leave 22 million more people uninsured by 2026 than under Obamacare, the nonpartisan Congressional Budget Office has predicted.

The bill, which is meant to repeal and replace Obamacare – officially known as the Affordable Care Act – is also expected to reduce the federal budget deficit by $321bn.

Last month, the House of Representives passed its own healthcare bill, which the CBO predicted would leave 23 million people uninsured by 2026.

The Senate plan significantly scales back Medicaid – a healthcare programme for the poor – repeals Obamacare’s taxes on the wealthy, and defunds Planned Parenthood for a year. Republicans have called Obamacare – officially known as the Affordable Care Act – “unsustainable”, citing skyrocketing premiums and the unstable health insurance marketplaces created by the law.

Several Republican senators have been waiting on the results of the CBO's analysis before making up their minds about the measure.

Since it was unveiled last week, the bill has been facing growing opposition from Republicans, with five already declaring that they cannot support the current version.

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With a majority of 52 senators, Senate Majority Leader Mitch McConnell can only afford to lose two ‘yea’ votes. In the event of a 50-50 tie, Vice President Mike Pence would cast the deciding vote.

The bill is not expected receive support from Democrats.

"Throwing 22 million Americans off of health insurance, raising premiums for older Americans, defunding Planned Parenthood and giving $231 billion in tax breaks to the top 2 percent is a cynical and immoral proposal," said Independent Senator Bernie Sander, who caucuses with the Democrats. "The reality is that this so-called ‘health care’ bill is nothing more than a massive transfer of wealth from working families to the very rich. All of us, including Republicans whose constituents depend on Medicaid to survive, must work together to see that this bill is defeated. Our job today is to improve the Affordable Care Act, not destroy it."

Those Republican senators that had already signalled before the CBO report include Texas Senator Ted Cruz, Kentucky Senator Rand Paul, Wisconsin Senator Ron Johnson, and Utah Senator Mike Lee.

A fifth GOP senator, Dean Heller of Nevada, has also pulled support from the bill.

"Currently, for a variety of reasons, we are not ready to vote for this bill, but we are open to negotiation and obtaining more information before it is brought to the floor", those four original GOP senators said in a joint statement. "There are provisions in this draft that represent an improvement to our current health care system, but it does not appear this draft as written will accomplish the most important promise that we made to Americans: to repeal Obamacare and lower their health care costs".

Those five holdouts mean that Senate leadership may face a tough road ahead in securing enough votes to pass the bill, sending it over to a joint committee with the House to iron out final legislation that could be passed in both chambers of Congress. The Senate was already looking to reach a fairly low bar, as far as major legislation is concerned, as they were using an obscure budget reconciliation process to pass the healthcare repeal without threat of Democrats filibustering the proceedings. If Democrats had the ability to filibuster the legislation, it is a near certainty that they would have exercised that tactic to effectively thwart any Republican efforts to replace Obamacare.

Outside groups also expressed dismay at the Senate bill's CBO score.

"Today, the Congressional Budget Office released a report on the ‘discussion draft’ of the Senate health care proposal, indicating that millions of people could lose their health insurance over time. This moment cannot pass without comment", Frank Dewane, the chairman of the US Conference of Catholic Bishop's Domestic Justice and Humane Development Committee, said in a statement. "The loss of affordable access for millions of people is simply unacceptable. These are real families who need and deserve health care. We pray that the Senate will work in an open and unified way to keep the good aspects of current health care proposals, to add missing elements where needed, and to not place our sisters and brothers who struggle every day into so great a peril on so basic a right".