Senior influenza experts have joined a growing backlash against the anti-flu drug Relenza which they fear could have potentially serious financial effects on the NHS and they claim is of unproven clinical effectiveness.

GPs are already saying that the National Institute for Clinical Excellence guidance on prescribing Relenza to 'at risk' groups will create a massive increase in workload.

This week two former members of the Department of Health's joint committee on vaccinations and immunisations, virologist Professor John Oxford and Dr John Watkins, the director of primary care at Gwent health authority, expressed concern that an anti-viral drug for influenza, which is a tenth of the price of Relenza, may be neglected in favour of the costlier treatment.

They say amantadine has been available in the UK for nearly 30 years and has a proven efficacy in the prevention and treatment of influenza A. It is now marketed as Lysovir and costs£2.40 per course, compared to£24 for Relenza.

But following last week's NICE decision the DoH has issued guidance to HAs which critics complain fails to mention amantadine.

NICE says the annual cost of the use of Relenza will be£2.3m£11.7m, but the manufacturers of Lysovir say that its use could save the NHS around£10m a year.

The distributor, Alliance Pharmaceuticals, says it has provided a dossier on the drug's efficacy to the DoH, but that the DoH has chosen to ignore it.

'We have written to ministers to point out that the guidance amounts to distortion of the market, ' said director Tony Booley.

Dr John Watkins said: 'There is a lack of awareness of this drug, and because it is off-patent there has not been the commercial incentive to have a big marketing push on it.'

Professor Oxford, professor of virology at St Bartholomew's and the Royal London School of Medicine, said improved surveillance means that scientists could tell which influenza strain was prevalent. 'In most years influenza A is the commonest cause of flu [and] Lysovir offers a simple and cost-effective answer.'

Some observers suspect that NICE may have come under political and industry pressure to approve the drug in order to avoid a potential flu epidemic this winter, and to pacify manufacturers Glaxo Wellcome after its threats to move its operations from the UK.

A DoH spokesperson said that the NICE brief was to examine evidence on Relenza specifically and not other anti-flu drugs.

Professor Joe Collier, editor of the Drug and Therapeutics Bulletin, has said that the evidence on the effectiveness of Relenza remains weak and that it does not support its prescribing to at-risk groups.

A Glaxo Wellcome spokeswoman said she could not comment on the claims about Lysovir without seeing technical data. She added that Professor Collier's comments reflected his own thinking: 'As far as we're concerned, the guidance on Relenza from NICE provides the physician with another tool in his armoury for the management of influenza.'