The NHS drugs watchdog is to face a judicial review in court for the first time over the guidance it offers.
Two firms have been granted the review over the way the National Institute for Health and Clinical Excellence reached its conclusion on Alzheimer's drugs.
NICE ruled NHS patients with newly diagnosed, mild Alzheimer's disease should not be prescribed the drugs.
Eisai and Pfizer said the process was unfair, but NICE said the claims were without foundation.
"Denying people in the early stages of this debilitating disease access to drug treatments is cruel and unethical"
NICE guidance in 2001 recommended donepezil, rivastigmine and galantamine - which can make it easier to carry out everyday tasks - should be used as standard.
But in November the watchdog announced people with newly diagnosed, mild Alzheimer's were exempt.
Eisai and Pfizer, who produce donepezil, also known as Aricept, have also asked "fully transparent working version of the calculations used in the cost effectiveness model for independent evaluation and comment".
Campaigners have repeatedly argued patients in the early stages of Alzheimer's should also have access to the £2.50-per-day drugs.
But NICE have so far refused to change their position.
The High Court granted the two firms permission for a judicial review on donepezil, but the eventual ruling will also apply to the other two drugs.
Pfizer managing director John Young said: "We accept that NICE has an important role to play on behalf of the NHS and our issue is not what NICE do, but how it has reached its decision."
And Neil Hunt, chief executive of the Alzheimer's Society, which will give evidence in the case, said: "Denying people in the early stages of this debilitating disease access to drug treatments is cruel and unethical.
"Our legal team will also demonstrate that the decision is fundamentally flawed."
But Andrew Dillon, chief executive of NICE, said: "We consider their claim without foundation and it will require us to divert energy and funding from the work we do to support patients and health professionals and get the most out of the resources available to the NHS."
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