Hundreds of children suffering from cystic fibrosis have written to Prime Minister Theresa May pleading for the life-changing drug, Orkambi.
More than a thousand letters are being delivered to Downing Street - about three quarters of them from children.
The National Institute for Health and Care Excellence has not recommended the drug on grounds of cost effectiveness.
Ministers have urged pharmaceutical firm Vertex to drop the price. Vertex said it wants to reach an agreement.
The Cystic Fibrosis Trust said the letter campaign was to ask Mrs May and her government to make sure discussions between Vertex and the NHS take place and are supported, so they can agree a fair price.
Seven-year-old Luis Walker, from Horam, near Heathfield, Sussex, wrote the first letter to Mrs May asking her to give him Orkambi.
His mother, Christina Walker, said: "One day he was sat at the table, he was struggling to breathe, coughing up phlegm and in desperation he said 'mummy, why can't won't they [sic] give me the medicine?'
"I said 'why don't you write a letter?'
"So he did, he wrote a letter. He felt good that he'd done it."
After Luis sent his letter, his friends, family, teachers and fellow patients decided to do the same.
Patients with cystic fibrosis cannot meet in person because of the risk of cross-infection, so word spread through the UK's online cystic fibrosis community via a private Facebook group.
Mrs Walker said: "It snowballed into a massive, emotional outpouring of desperation."
Orkambi has been shown in clinical trials to improve lung function and respiratory symptoms in people with cystic fibrosis - a genetic, life-shortening condition that causes fatal lung damage.
David Ramsden, chief executive of the Cystic Fibrosis Trust, said: "Significant advances in medical science mean new precision medicines are available or in development that have the potential to transform the lives of people with cystic fibrosis.
"We urge the prime minister to ensure that they are available for the thousands of children and adults who have been waiting for far too long."