David Cameron's "friends and family" satisfaction survey of hospitals is regarded as unnecessary by the public and should be dropped in its current guise because it could be misleadingly negative about the NHS, according to a government-commissioned evaluation.

Under the prime minister's plan, patients and staff will be able to score every hospital and GP in England on whether they would be happy for their loved ones to use the service. Cameron personally launched the scheme, which comes into place this month, claiming that it could act as a "flashlight" on the NHS's failings.

However, a report from Ipsos Mori found a lack of interest among both staff and patients in the scheme. It also highlighted widespread concerns that the methodology subsequently chosen by the government for the survey would be misleading and advised that it should be dropped.

One particular concern was that a response would only be recorded as positive when a person said they were "extremely likely" to recommend an NHS service. Those who would be "likely" to recommend a hospital are to be regarded as offering a neutral response, and all other responses – including those who say they are unsure – will be recorded as negative. wThe Ipsos Mori report, commissioned by the Department of Health, said one focus group "got very focused on why 'likelys' were counted as neutrals and 'neither/nors' as detractors – which they felt did not accurately represent what the patients had meant when responding to the survey".

The report concluded that the methodology "should be dropped" because "it is strongly resisted by the public, and rejected by provider staff and some opinion formers as misrepresenting the survey responses it is trying to code. Providers felt this would undermine its credibility and make it far harder to engage patients and staff in improving services."

The Ipsos Mori report added: "The public did not appear convinced that this was a necessary measure – with comments about the cost of implementing, and also that they would tend to rely on GP recommendations, or other data, such as mortality rates – to decide which hospital to go to. This emphasises the point that, if the public are to engage with this measure, the scoring mechanism does need to be simple, readily explainable and seen as credible."

The chosen design of the friends and family survey has raised fears that the new scheme is being used to undermine NHS hospitals just as people are being offered the chance to choose a private provider for operations.

The controversial Health and Social Care Act encourages private providers to offer treatment to NHS patients. But as NHS hospitals currently offer the vast majority of services, there are concerns that the survey could work to undermine the NHS.

The shadow health secretary, Andy Burnham, said: "It is essential that any new NHS target carries the confidence of patients and staff. That will not be possible if there is even the merest whiff of political manipulation."

A health department spokesman said a more generous system would have given a majority of trusts highly favourable results, so differentiation was needed. She said: "We drew on a number of pieces of research to inform the test. We needed a scoring system that was simple, but not so simple that it failed to expose variation."