The court "finds the individual mandate, unmoored from a tax, is unconstitutional", U.S. District Court Judge Reed O'Connor, an appointee of President George W. Bush presiding in Texas, wrote in his opinion.
U.S. District Judge Reed O'Connor in Fort Worth agreed with a coalition of 20 states that a change in tax law past year eliminating a penalty for not having health insurance invalidated the entire Obamacare law.
'Wow, but not surprisingly, ObamaCare was just ruled UNCONSTITUTIONAL by a highly respected judge in Texas. They contended that overturning the law would throw millions off health insurance rolls by reversing Medicaid expansion, ending tax credits that help people and empowering insurers to once again deny coverage based on pre-existing conditions.
This is a breaking story and will be updated.
"As I predicted all along, Obamacare has been struck down as an UNCONSTITUTIONAL disaster", President Trump tweeted following the ruling.
Twenty Republican-led states had argued the Supreme Court cast Obamacare as a package deal, with the mandate to hold insurance - or else pay a tax - tethered to the law's goodies.
A federal Fort Worth judge filed a decision in a national Obamacare ruling on Friday, casting doubt on coverage for millions of Americans.
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An appeal is expected, and that could go as far as the Supreme Court.
With the mandate's repeal, the U.S. Supreme Court's 2012 ruling that Obamacare was constitutional could no longer be considered valid.
The Affordable Care Act was passed in 2010 with the aim of providing "near-universal" health insurance coverage and to "lower health insurance premiums", said the background to O'Connor's decision.
California and other states ruled against by the judge will challenge the decision by appealing in the United States Court of Appeals for the Fifth Circuit.
White House press secretary Sarah Huckabee Sanders said in a statement that the decision "vindicates President Trump's position that Obamacare is constitutional".
Democratic leaders, meanwhile, reacted harshly to the decision. "Pending the appeal process, the law remains in place".
"Today's ruling is an assault on 133 million Americans with preexisting conditions, on the 20 million Americans who rely on the ACA's consumer protections for health care, and on America's faithful progress toward affordable health care for all Americans", California Attorney General Xavier Becerra said in a statement, Bloomberg reports, adding that a "spokeswoman for Becerra said an appeal will be filed before January 1".