GCXZ

Those who answered extremely likely were categorised as "promoters" of the service, and an overall score was calculated by weighing their responses against "detractors" who were extremely unlikely, unlikely or unsure.

For example, a ward or unit which had 100 responses, including 75 "promoters" and 25 "detractors", would be given a score of 50.

One emergency department, at Chase Farm Hospital in London, and 36 of the 4,500 wards surveyed in England received a greater number of negative scores than positive ones in June, figures showed.

But although the survey included responses from 400,000 people in the UK, critics said widely varying response rates meant that hospitals could not be accurately compared against one another.

Many of the wards with the highest and lowest ratings had received fewer than ten responses, and some were given either the best or worst possible score based on a solitary positive or negative response.

NHS England will update the figures each month, and by the end of next year the test is expected to be rolled out in GP practices and community and mental health services.

Mr Cameron, who is on holiday in Portugal, said: "I am determined to give patients a far greater voice within the NHS as a way of highlighting the best and worst of care within our hospitals.

"With the 'Friends and Family' test, we now have a single measure that looks at the quality of care across the country."

Deputy Prime Minister Nick Clegg told BBC Radio 5 Live: "What we are doing is introducing a new culture of greater transparency and accountability in the NHS."

Challenged over the poor ratings some wards received in the family and friends test, he Clegg said: "You can't make sure the NHS is there for everybody at very high standards unless we are open and candid with each other about what is not perfect about it, just as we celebrate the great work done in the NHS."

But the data were criticised by Katherine Murphy, Chief Executive of the Patients Association, who said: "The data is difficult and confusing to navigate from a patient's point of view – we are not sure how this is going to help patients make choices."

Tim Kelsey, NHS England's National Director for Patients and Information, admitted that low response rates could have a "dramatically disproportionate" impact on scores but insisted the data contained "home truths" which must be addressed.

Lord Phil Hunt, Shadow Health Minister, added: "It is vital that patients are able to give their views on what is happening in hospitals across the country. But it is also important that the results are balanced and meaningful.

"The Government needs to improve the response rates from patients, but it also needs to publish the underlying data so that proper judgements can be made on the performance of hospitals."