Concerns have been raised over proposed legislation that could give GPs the power to charge patients for services.
The issue was raised by Labour leader Ed Miliband (pictured), who also said the government's controversial health reforms could mean the NHS is subject to European Union (EU) and UK competition law, which could see hospitals fined up to 10% of their income.
"This Bill is a Pandora's Box. The more people look at the detail, the more profound and worrying the implications appear to be for the NHS," he said at a press conference at Labour Party headquarters in London.
Under the laws, the health secretary would no longer be responsible for deciding what services the NHS should provide, with this instead transferred to GPs.
Mr Miliband said: "I take David Cameron at his word when he says he has no intention of undermining the principle of providing healthcare free at the point of need. But I have never heard the government explain what the effect of this transfer of power over charging to GPs will be, nor seek to defend it."
He also added that ministers had conceded during the committee stage of the Bill that the NHS would increasingly be liable to both EU and UK competition law as a result of the government's market reforms.
The result would be that hospitals could be fined up to 10% of their income if they were found to have breached the legislation, which would deter hospitals from working with each other.
At the same time, he said that hospitals would also be subject to insolvency laws, which would mean that they could go bust in the same way as commercial businesses.