A decision by the government's drugs watchdog to restrict the use by the NHS of Alzheimer's medication is to be challenged in court.
The drugs are reserved for those with moderate Alzheimer's
Two drug companies plan to apply for a judicial review of the way the National Institute for Health and Clinical Excellence reached its conclusion.
NICE ruled NHS patients with newly diagnosed, mild Alzheimer's disease should not be prescribed the drugs.
Eisai and Pfizer claim the process leading to this guidance was unfair.
They claim many of NICE's conclusions on the Alzheimer's treatments - which include donepezil, rivastigmine and galantamine - cannot be supported legally or are "irrational".
They want NICE to withdraw the recommendations and postpone issuing its final guidance, which is due on 22 November.
Eisai and Pfizer, who produce donepezil (also known as Aricept) are also asking the institute to disclose a "fully transparent working version of the calculations used in the cost effectiveness model for independent evaluation and comment".
They would like NICE to develop a new guidance using "both a more accurate cost effectiveness model and data".
Dr Paul Hooper, managing director of Eisai Limited, said: "We are deeply concerned about the way that NICE's decision on treatment recommendations for early Alzheimer's disease was reached.
"A judicial review is now the only option remaining to us to ensure that NICE reconsiders how it arrived at such flawed conclusions."
If the judicial review goes ahead it would be the first time a NICE decision has been contested at this level.
NICE, whose guidelines cover England and Wales, has 14 days to formally respond to Eisai's written proposals, after which Eisai may apply to the High Court for permission to proceed to judicial review.
Campaigners have repeatedly argued patients in the early stages of Alzheimer's should also have access to the £2.50-per-day drugs.
Neil Hunt, chief executive of the Alzheimer's Society, said: "It's great news that NICE will be challenged in court. NICE holds the fate of thousands of people's lives in its hands and it is only right that it is brought to account."
The society is co-ordinating a number of protest marches across the country calling for doctors to have greater flexibility in prescribing Alzheimer's treatments.
Last month, NICE rejected their appeal saying studies showed the drugs "did not make enough of a difference".
Andrew Dillon, chief executive of NICE, said: "We will respond to Eisai's letter and act appropriately in any court proceedings which may follow.
"In the meantime, we will publish our recommendations, both on the best ways of caring for people with all forms of dementia and on the use of drugs for treating Alzheimer's disease, on 22 November 2006.
"This will be the first time that health and social care advice has been combined in a single guideline and we believe it is in the interests of those affected by this distressing condition to make it available now."