The drugs industry is planning to take court action over new limits on the price the NHS will pay for medicines.
The Association of the British Pharmaceutical Industry (ABPI) said it has applied for a judicial review after a Government shake-up over how much NHS England would be prepared to shell out.
In April, patients were told they could face delays accessing drugs if they cost £20 million or more in any of the first three years of their rollout across the NHS.
Even when a drug has been approved by the National Institute for Clinical Excellence (Nice) - which already has strict rules on affordability - bosses at NHS England can now slow down its delivery to patients for three years or more.
At present, the NHS has 90 days to make Nice-approved drugs available.
NHS England said the scheme meant it had more time to negotiate with drug firms to lower the cost of the treatments.
But health charities have objected strongly to the move and said NHS England will also restrict access to certain groups of patients.
In a statement, the ABPI said "reluctant" legal action against Nice was necessary because of the introduction of the £20 million limit - even when medicines have already been assessed as cost-effective. The body said one-in-five new medicines will be impacted.
The ABPI is also seeking to reverse changes to the assessment of medicines for very rare diseases, which it believes to be "inappropriate and unworkable".
Mike Thompson, ABPI chief executive, said: "These new arrangements will delay access to cost-effective medicines and deny treatments to patients suffering from rare diseases.
"After many months of raising concerns with Nice, NHS England and the Department of Health and offering to work constructively on alternative proposals, we have applied to formally challenge these proposals in court.
"We believe this to be the right course of action due to the potential damage these changes will cause to NHS care and on our ability to research, develop and use new medicines here in the UK.
"We hope that the Government will reverse the changes and work with us to find a solution that works for everyone."