Evidence to suggest the NHS care records service will save thousands of lives is 'patchy', the government has admitted.

The assertion, made by health secretary Patricia Hewitt to counter a campaign encouraging patients to opt out of the electronic records system, is based on a two-month study of two Greater London hospitals in 1998, it has emerged.

The study found that about 10 per cent of patients on medical wards had experienced an adverse event, of which half were preventable. But it warned that no extrapolations could be drawn from the findings.

In a letter to Conservative MP Stephen O'Brien, seen by HSJ, Lord Hunt said the report had provided the basis for the government's support for the system. The health minister admitted: 'Hard data is patchy, and the figures are difficult to quantify precisely.'

The letter also reveals that there will be an extensive programme to inform the public about the electronic records system and engage NHS staff. The programme will be delivered by primary care trusts but no extra funds will be available.

London PCT Policy Forum chair Robert Creighton said: 'It's potentially frustrating for local organisations to have to pick up the costs of a central initiative.'

A DoH spokesman said: 'Connecting for Health has put in place an extensive communications framework that PCTs can use free of charge. Additional local activity by PCTs is funded by themselves.'