US President Barack Obama's controversial health care law called Obamacare gained a huge boost after the Supreme Court, in a 6-3 decision on Thursday, gave its approval to the system of granting federal tax credits to eligible Americans throughout the nation.

Issuing the majority opinion, Chief Justice John Roberts firmly defended Obamacare, a law that will serve as a hallmark of the Obama's presidency for generations to come.

"Congress passed the Affordable Care Act to improve health insurance markets, not to destroy them," Roberts wrote in the majority opinion. "If at all possible, we must interpret the Act in a way that is consistent with the former, and avoids the latter."

The White House quickly lauded the Supreme Court decision. "Five years ago, after nearly a century of talk, decades of trying, a year of bipartisan debate, we finally declared that in America, health care is not a privilege for a few but a right for all," Obama said from the White House. "The Affordable Care Act is here to stay"

However, critics claimed that what the Supreme Court did was an attack on the "rule of law."

According to the Citizens' Council for Health Freedom (CCHF), which advocates health care choices, the Supreme Court's "disastrous decision is a terrible attack on the rule of law because it sends the message that the rule of law does not matter; whoever has most power, the strongest attorneys and the biggest voice wins."

CCHF president and co-founder Twila Brase said "without the rule of law, it becomes the rule of power—all up to interpretation. Claims of intent and outside interpretations of intent will now rule, not the actual words written in the law. When we start allowing the loose interpretation of law based on after-the-fact claims of intent, the foundation of the rule of law crumbles."

Brase claimed that the Supreme Court did not actually rule on what is actually written in the law.

"And sadder still, the ruling means that citizens in 37 states, including the 6.5 million Americans receiving Obamacare subsidies, didn't regain their health freedom today. They will remain trapped in a government health care system of mandates and penalties that does not work, doesn't look out for their best interests, makes health care unaffordable, and puts their private medical data at risk," she said.

Republican leaders joined in the protest, saying that despite the ruling, Obamacare is broken.

US Senate Majority Leader Mitch McConnell said the ruling won't change Obamacare's broken promises. "Today's ruling won't change Obamacare's spectacular flops, from humiliating website debacles to the total collapse of exchanges in states run by the law's loudest cheerleaders," he said.

Speaker John Boehner said the Supreme Court decision does not change the fact that Obamacare is "fundamentally broken."

"And we will continue our efforts to repeal the law and replace it with patient-centered solutions that meet the needs of seniors, small business owners, and middle-class families," he said.

Obamacare, or the Patient Protection and Affordable Care Act, is the largest overhaul of the US healthcare system since the 1960s.

Its principal objective is to extend health insurance coverage to some of the estimated 15 percent of the US population who lack it. These are the people who receive no coverage from their employers and are not covered by US health programmes for the poor and the elderly.

Towards this end, Obamacare requires all Americans to have health insurance. To facilitate this, the Obama administration offers subsidies to make health insurance more affordable.

The law, enacted in 2010, also requires businesses with more than 50 full-time employees to offer health coverage. This provision was delayed until this year to allow more time for compliance.

The law creates marketplaces—with websites similar to online travel and shopping sites—where people can compare prices as they shop for health insurance coverage.

Obamacare was meant to slow the growth of US healthcare spending, which is the highest in the world.

The basic objection to the law is that it imposes too many costs on business, with many describing it as a "job killer."

Critics also describe it as an unwarranted intrusion into the affairs of private businesses and individuals.