US President Barack Obama has spoken out against a Supreme Court ruling that removes limits on the amount that companies, organisations and trade unions can donate to political campaigns.

Five of the Supreme Court's nine judges voted to do away with the restrictions in the interests of freedom of speech.

But the president has condemned the decision as a victory for "big oil", Wall Street and health insurance companies and said it put them in a position to drown out the voices of average Americans. He has vowed to come up with a powerful response to the ruling.