Barack Obama has delivered a surprisingly strong warning to the US Supreme Court, saying that it would be guilty of an “unprecedented” case of “judicial activism” if it overturned his signature healthcare law.

Last week America’s highest court heard arguments for and against the healthcare reform law ? officially known as the Affordable Care Act but derided as socialist “Obamacare” by Republicans ? but the judges are not due to release a ruling until late June.

On Monday the president said he was “confident” the law would be upheld, questioning how an “unelected group of people” could overturn a law approved by legislators.

“I’m confident that the Supreme Court will not take what would be an unprecedented, extraordinary step of overturning a law that was passed by a strong majority of a democratically elected Congress,” Mr Obama said during a press conference with Felipe Calderon and Stephen Harper, the visiting leaders of Mexico and Canada respectively.

The president’s comments were all the more striking since the decision is not yet known.

“For years, what we have heard is the biggest problem on the bench was judicial activism, or a lack of judicial restraint, that an unelected group of people would somehow overturn a duly constituted and passed law,” he said.

His talk of “judicial activism” echoes Republicans’ oft-voiced concerns about justices acting on political considerations.

Overturning the law would be a blow to Mr Obama ahead of the November presidential election, although it could prove so incendiary that it might mobilise disillusioned liberals.

Whatever the result, it will probably create further difficulties for Mitt Romney, the favourite to win the Republican nomination, because it will attract new attention to the health reforms he instituted as governor of Massachusetts, which formed the blueprint for Mr Obama’s initiatives.

The president, a former constitutional law professor at the University of Chicago, said there was no legal reason for throwing out the law.

“That is not just my opinion ? that is the opinion of a whole lot of constitutional law professors and academics and judges and lawyers who have examined this law even if they are not particularly sympathetic to this particular piece of legislation or my presidency,” he said.

Mr Obama had not publicly commented on the Supreme Court hearings previously and the White House will not discuss contingency plans if the court overturns all or part of the law.

The most controversial aspect of the law is the “individual mandate”, a requirement that almost all Americans buy health insurance, often with subsidies, or pay a penalty. The requirement is considered crucial for making the entire health overhaul work by ensuring that everyone, not just the sick, is covered.

Conservative justices on the court, and the swing voting judge Anthony Kennedy, signalled discomfort with the individual mandate during the hearings last week, raising the spectre that they could rule it unconstitutional.

But without an individual mandate, “you cannot have a mechanism to ensure that people with pre-existing conditions can actually get healthcare”, Mr Obama said, adding that the case was not only an economic and legal one, but a human one too.

“I hope that is not forgotten in this political debate,” he said.

All of the Republican presidential candidates, including Mr Romney, have promised to repeal the law if elected.

Republicans, particularly the conservative Tea Party movement, have railed against the healthcare reform, which would extend coverage to 30m uninsured Americans, as a symbol of big government intrusion into their lives.