The US Supreme Court today considered whether to strike down Barack Obama’s entire healthcare reform law, as the president’s landmark domestic achievement looked increasingly in peril.

On the third day of hearings into ‘Obamacare’ – the reforms designed to provide health coverage to millions of uninsured Americans – several of the nine justices signaled that they believed the fate of the whole law rested on the constitutionality of its most controversial element.

At the heart of Mr Obama’s reforms is the individual mandate, a provision that forces Americans to buy health insurance or else face a financial penalty. The court’s conservative justices are deeply opposed to the regulation, which they believe represents an overreach by the federal government.

In today’s hearings, opponents of the law argued that if the individual mandate is struck down then all other aspects of the Affordable Care Act must be struck down with it.

Paul Clement, a lawyer representing 26 Republican states opposed to the reforms, told the court: “If the individual mandate is unconstitutional, then the rest of the act cannot stand.”

His argument met a warm reception from the more conservative judges, with Justice Anton Scalia saying “if you take the heart out of this statute” then all its other provision must also fall.

Edwin Kneedler, a lawyer for the Obama administration, urged the court to reject his opponents’ “sweeping” claims that all elements of the law would be scuppered by the loss of the individual mandate.

He said that other provisions – including an expansion of the Medicaid programme for treating the very poor and subsidies on health insurance for those on low incomes – represented traditional areas of government intervention and could stand regardless of the fate of the mandate.

His argument was supported by the court’s more liberal judges. Justice Ruth Bader Ginsburg said she favoured “a salvage job” which would allow Congress to decide the fate of the act’s surviving elements, rather than leaving them in the hands of unelected judges.

Jeffrey Toobin, one of the country’s most influential legal experts, said the tone of the hearing indicated serious dangers for the president’s reforms.

“There’s considerable sentiment on this court that it’s all one big package and it needs to be got rid of altogether, which is a stunningly bad turn of events for the Obama administration,” he said.

The court is expected to give its ruling in June.

Today’s discussion may prove only theoretical if the mandate is deemed constitutional, but in a critical session yesterday five of the nine justices seemed to indicate they were prepared to knock down the controversial regulation.

Both opponents and advocates of the law focused their attention on Justice Anthony Kennedy, the Reagan-appointee who is seen as the court’s “swing vote” and who yesterday seemed to focus his criticisms on the administration’s case.

At one stage he interrupted Donald Verrilli, Mr Obama’s solicitor general, to apparently express alarm at the scale of the consitutional implications of the mandate.

“When you are changing the relation of the individual to the government in this, what we can stipulate is, I think, a unique way, do you not have a heavy burden of justification to show authorisation under the Constitution?” he asked.

As they emerged from Tuesday’s hearing, Republican senators and attorney generals from the 26 states opposing Mr Obama’s reforms seized on Justice Kennedy’s questions as evidence they were closing in on their goal..

Greg Abbott, the Attorney General of Texas, told the Telegraph: “Justice Kennedy was very clear in his questions that he was concerned about the law’s intrusion into individual liberty and the precedent it would set for the limits on congressional power.”

Liberal groups struck a more subdued note but Elizabeth Wydra, a lawyer for 500 state legislators who support the reforms, said that conservatives had yet to lock down the votes of either Justice Kennedy or Chief Justice John Roberts.

“I still see a way to gather a majority for upholding the constitutionality of the mandate and I think that the conservative can count on three justices but I don’t think they can count beyond that,” she said.