The prospect that patients may be allowed to top up National Health Service treatment with private care came a step closer on Tuesday in a move that pro-market think-tanks said marked the end of the NHS as a system where treatment was based entirely on need, not ability to pay.

Alan Johnson, health secretary, announced a four-month review of the issue. He said Mike Richards, the national clinical director for cancer, would examine the current policy, which stipulates patients have to pay for the whole of their episode of care – that is, go entirely private – if they purchase cancer or other drugs that are not funded by the NHS.