The 'founding principles of the NHS' are to be revisited after the government launched a review of the ban preventing patients making 'top-up' payments for drugs.

Health secretary Alan Johnson has asked the Department of Health's national clinical director for cancer Mike Richards to undertake the three-month review, which will focus on drugs rather than operations. His recommendations must take account of both the importance of giving patients "choice and personal control" and "the need to uphold the founding principle of the NHS that treatment is based on clinical need not the ability to pay".

Mr Johnson said he did not want to pre-empt the outcome but that the NHS needed "guidance that is actually up to date with what's happening now in the NHS, that is fair to everyone and that actually resolves some of the problems raised".

As recently as December, Mr Johnson told the Commons that any change to the top-up ban would imply "the end of the founding principles of the NHS" and make a "nonsense" of the 2000 NHS Plan.

The change in tone is understood to have been directed by Number 10, following embarrassing cases of cancer patients "shut out" from NHS care after purchasing additional drugs.

The three-month review - announced by Mr Johnson in the Commons on Tuesday - was greeted with a measured welcome from MPs and NHS leaders.

It followed a paper by the NHS Confederation and Institute for Public Policy Research outlining a "middle way" on top-up payments for drugs. This would centre on those deemed only "marginally effective" by the National Institute for Health and Clinical Excellence and outside its cost-effectiveness threshold.

NHS Confederation policy director Nigel Edwards said such a change would have consumer protection implications. Patients would need independent advice to prevent them from falling prey to drug advertisers promising unrealistic results. The IPPR has proposed an advice service could be funded by a "top-up premium".

NHS Alliance chair Michael Dixon said the rules were inconsistently applied and clashed with the recent emphasis on personalisation. "We are moving out of the era of 'doctor knows best' and whilst we wouldn't want patients to take treatments which might damage them, we have to allow patients to make more decisions about what they think is appropriate for them," he said.

But Association of Directors of Public Health president Tim Crayford warned that relaxing the rules could exacerbate health inequalities.

"There is the potential for a shift in policy to affect health inequalities - the tendency for the better off to live healthier and longer lives."

However, he said it "seemed unfair" that patients wanting to top up their care with additional treatments risked excluding themselves from NHS care altogether.

Unison head of health Karen Jennings reacted to news of a likely change in policy with anger. "We are overwhelmingly against a change in top-up policy," she said. "It opens up the opportunity for a two-tier service."

Unison's staunch reaction comes in the wake of the union's dispute with Monitor over its interpretation of legislation that limits the proportion of income that foundation trusts can make from private patients.

Monitor responded to Unison's challenge this week by launching a consultation into alternative interpretations of the cap.

It warned that if the cap were too widely defined, activities such as the provision of laboratory services to another NHS hospital could be deemed as generating private patient income if the purchasing hospital treated private patients.

In 2007-08, 63 foundation trusts declared a total of£165m in private income (see table). It is not known how much extra they earned but they were not obliged to report under the current rules.