Vertex Pharmaceuticals Inc. suffered a setback Wednesday when a British regulatory agency recommended that Orkambi, the company’s new cystic fibrosis treatment and potential top-selling product, was too expensive for coverage by government insurance.

The decision by the National Institute for Health Care Excellence, which does cost-benefit analyses for England’s National Health Service, sent Vertex’s stock down 7.6 percent to $80.15. Britain is one of the world’s largest prescription drug markets.

The so-called approval consultation document from the agency, known as NICE, is just the first step in the reimbursement process. NICE officials will now hear feedback from Vertex and other parties in the coming weeks before making a final decision.

Vertex executives said they will continue their dialogue with NICE but complained the agency used a different process to evaluate Orkambi, a two-drug combination, than it used to review an earlier cystic fibrosis drug. The new process fails to recognize benefits to patients taking the treatment, such as weight gain and fewer lung infections, they said.

“We are not surprised,’’ said Vertex spokeswoman Dawn Kalmar. “We expected this from NICE. We don’t think the process that is currently being used captures the full benefits of Orkambi. But we will continue to work with NICE to make Orkambi available to patients.’’

The National Health Service would also have to sign off on the medicine before it can be made available to about 2,750 eligible patients aged 12 and over in Britain who have the most common genetic mutation for cystic fibrosis, a life-threatening lung disease that causes the buildup of mucus in lungs and the digestive system and can cause recurring infections.

European regulators gave Vertex marketing authorization for Orkambi last fall, but individual countries must separately approve the price of the therapy in a fragmented process.

NICE said its evaluation panel concluded that Orkambi’s benefits weren’t sufficient to justify the National Health Service paying Vertex’s proposed price of about $150,000 a year per patient.

“When compared to the current standard of care, the benefit [Orkambi] offered was modest and comes at a considerable cost,’’ Carole Longson, director of the NICE Centre for Health Technology Evaluation, said in a statement. “We have to recognize that the NHS is a finite resource, and we can only recommend treatments for routine funding that are both clinically effective and represent good value for money.’’

While other drugs treat cystic fibrosis symptoms, such as coughing and lung infections, Vertex said, Orkambi and another approved drug, Kalydeco, which treats a different genetic mutation, are the only medicines that address the underlying cause of the disease.

Orkambi was approved by the Food and Drug Administration last July for sale in the United States, where there are more than 8,500 eligible patients. The drug costs $259,000 per patient annually in the United States, a price that has drawn criticism from consumer groups.

NICE spokeswoman Liz ­Adelanwa said “there’s still an opportunity to work with the [drug] manufacturer’’ before a final decision is made on Orkambi. “It’s a case-by-case situation,’’ she said. “It’s really about, is it effective compared to the standard of care, and is it cost effective?’’

A statement from NICE said its evaluation committee found the Vertex medicine did help reduce symptoms that required hospitalization. “However, the benefits to lung function — the test to see how people with cystic fibrosis are improving overall — were modest,’’ it said.

Orkambi is currently on the market only in the United States, where the government does not approve drug prices, and Germany, where government officials have yet to complete their price negotiations with Vertex. But stock analysts project that it could eventually become one of the best-selling drugs globally, generating more than $1 billion in annual sales.

The largest cystic fibrosis patient group in Britain criticized NICE’s recommendation against Orkambi.

“This decision by NICE today will come as a blow to thousands of people that desperately need Orkambi,’’ Ed Owen, chief executive of the Cystic Fibrosis Trust, said in a statement. “It would be ethically unforgiveable if people with cystic fibrosis were treated like pawns in a bigger battle between the NHS and Vertex over price and longer term impact.’’

Owen called on Vertex and the National Health Service to find a way to make sure eligible patients in England get access to the drug.

Robert Weisman can be reached at robert.weisman@globe.com. Follow him on Twitter @GlobeRobW.