The drug, which has transformed the lives of people with the fatal disease, has been on the market in Europe and the US for three years.
The NHS ruled this month it will not fund Selexipag to treat the condition which raises blood pressure in the vessels between the heart and lungs.
Dr John Wort at London’s Royal Brompton Hospital said: “Potentially hundreds could benefit from this life-saving drug which reduces hospital admissions and therefore cost to the NHS.
“Selexipag is a step-change in treatment as it is the first oral medicine targeting a particular pathway.”
Dr Iain Armstrong, chairman of the Pulmonary Hypertension Association, said: “We are bitterly disappointed with the decision. An urgent review is needed.”
Patients have to rely on nebulisers.
Only half of the 2,600 patients in England survive four years.
The drug relaxes the blood vessels in the lungs to allow blood to flow.
In a trial of 1,156 patients in the New England Journal of Medicine, Selexipag halved hospitalisations.
Janet Coe from Hatfield, Hertfordshire, could not walk up a slope in 2010.
Mrs Coe said: “I was doubled up trying to breath. I couldn’t carry on.” She was diagnosed in 2012 and put on Selexipag in 2014. She said: “It transformed my life.”
NHS England said the drug is not approved by Nice and added: “It was not found to be effective enough to justify special treatment but will be reassessed in November.”