Leading doctors have called on Health Secretary Patricia Hewitt to ensure patients can get access to two new brain tumour treatments.
Treatments are designed for aggressive tumours
In a letter to the government, 36 clinicians say the National Institute For Health and Clinical Excellence (NICE) is ignoring patients' needs.
They say the treatments, temozolomide and carmustine implants, are a major advance, and value for money.
NICE initially rejected them - but said no final decision had been made.
The therapies are designed to treat a particularly aggressive form of brain cancer called a high-grade glioma.
It has been estimated that temozolomide alone could potentially benefit 800 patients a year in the UK.
The move will intensify the debate over whether it is right to deny potentially beneficial treatments to NHS patients.
Mrs Hewitt has intervened in a row about access to NHS drugs once before, asking to see the evidence upon which a primary care trust had decided to deny the breast cancer treatment Herceptin to a woman in Staffordshire.
Following her intervention, North Stoke Primary Care Trust reversed its decision.
NICE is due to begin its final assessment of the glioma treatments at the end of the month.
However, a preliminary recommendation from the NICE Appraisal Committee concluded that the treatments should not be widely used for the treatment of newly diagnosed high-grade gliomas.
In their letter to Mrs Hewitt, the consultants and specialists say the committee has failed to grasp the complexities of what they say is a particularly specialised area.
Professor Roy Rampling, an expert in brain tumours based at the Western Infirmary in Glasgow, said: "These treatments are the biggest breakthroughs in treating brain tumours in 30 years, and are standard care in many other developed countries.
"NICE is ignoring the evidence - and the needs of patients who are diagnosed with a rapidly fatal illness."
The letter calls on NICE to withdraw its current recommendation, and for Mrs Hewitt to ensure that the treatments are available for all brain tumours who would benefit from them.
It argues that if the drugs are not made widely available then it will make it more difficult to achieve targets set out in the government's cancer plan.
Professor Rampling said: "NICE simply isn't doing its job properly.
HIGH GRADE GLIOMA
Annually there are around 3,500 new cases of brain tumour in England and Wales
They are classified from grade I (slow growing and benign) to grade IV (fastest growing)
Grade III and IV tumours, which account for 50%-60% of all primary brain tumours, are classed as high-grade gliomas
There are several types, all of which are associated with severe disability, and a high risk of death
"Telling patients they have a malignant brain tumour is a difficult thing to do; we now might be in the invidious position of explaining that there are treatments available, but the flawed decision by NICE means their chances of getting them on the NHS are pretty much zero."
A consortium of brain tumour organisations, including Brain Tumour UK, the Samantha Dickson Research Trust and the International Brain Tumour Alliance, are also campaigning for improved access to the drugs.
Angela Dickson, a spokesperson for the consortium, said: "We're the ones who get calls from patients asking what to do when they've been told there's no funding for drugs which can increase their life span and slow the progress of their tumours.
"The current system is unfair and unacceptable."
In a statement, NICE said all representations would be considered before final guidance was issued.
It stressed there was no ban on prescribing licensed drugs that have not yet been appraised.
A Department of Health spokesperson said: "NICE is an independent body and we trust them to produce clinical and cost effective guidance to the NHS based on the latest available evidence."