The ruling could transform the way political campaigns are fought
The US Supreme Court has rejected long-standing limits on how much companies can spend on political campaigns.
The ruling is likely to change the way presidential and congressional campaigns are funded, including this year's crucial mid-term elections.
The court's 5-4 vote ends a 20-year ban on businesses using money from their own funds to pay for campaign ads.
But US President Barack Obama condemned the decision, pledging to work with Congress for a "forceful response".
He said the court had "given a green light to a new stampede of special interest money in our politics".
"It s a major victory for big oil, Wall Street banks, health insurance companies and the other powerful interests that marshal their power every day in Washington to drown out the voices of everyday Americans," he said in a statement.
Supreme Court Justice Anthony Kennedy said the prohibition of direct contributions from companies and unions to political candidates was a form of censorship.
"We find no basis for the proposition that, in the context of political speech, the government may impose restrictions on certain disfavoured speakers," he wrote.
His view was mirrored by that of Chief Justice John Roberts who said that upholding the limits on corporate campaign spending would have restrained "the vibrant public discourse that is the foundation of our democracy".
But Justice John Paul Stevens disagreed strongly, saying that the court's ruling threatened "to undermine the integrity of elected institutions around the nation".
He was joined in his opposition to the ruling by the court's three other liberals, including Justice Sonia Sotomayor, who was appointed by President Barack Obama.
The ruling also overturned part of a landmark campaign finance bill which banned unions and companies from paying for political ads in the closing days of an election campaign.
The Supreme Court also said that any campaign adverts that were not paid for by the candidate or their party must be clearly marked with the name of the sponsor.
The decision comes less than 10 months before the congressional mid-term elections.