Thousands of British women with advanced breast cancer may be denied a drug that can keep the disease in check, campaigners fear.
Research presented at the world's biggest cancer conference in Chicago showed that women taking Ibrance (palbociclib) with another common drug lived almost twice as long without their cancer getting worse as those given standard treatment.
Typically, they enjoyed 24.8 months without their cancer progressing compared with 14.5 months for women on usual treatment. Some women had much better results.
Around 27,000 women in the US have so far received the drug after regulators gave it fast-track status because of the compelling results of clinical trials.
But it has not yet been approved in Europe and charities fear the National Institute for Health and Care Excellence (Nice) will turn it down anyway for use on the NHS.
Delyth Morgan, chief executive of Breast Cancer Now, said Ibrance was part of a new class of drugs and a "significant step forward" for treating women.
But she said: "We're really concerned about this getting through the Nice appraisal system because what we've seen over the last seven years is that new breast cancer drugs are just not getting through the process."
She said she was worried it would not "get through to the women who could benefit - the majority of women with advanced breast cancer".
Ibrance is for women with oestrogen receptor-positive breast cancer or HER2 negative breast cancer that has spread around the body.
This affects around 25,000 women in UK, who are usually treated with 20p-a-day aromatase inhibitors (AIs). Baroness Morgan said comparing Ibrance - which costs around 10,000 dollars a month - with such cheap AIs means it will not be approved.
"If it were to be appraised now, our system would compare it to an existing and cheap generic medicine, despite it being the first drug of its kind," she said.
"As such, it would be near-impossible for this new patented drug to show it can be cost-effective."
She said she was "absolutely" convinced that Nice would turn it down, even though it offered "invaluable" extra time to patients.
"It could mean seeing the birth of your grandchild, being around long enough to see your children graduate," she said. "We're talking about an average of 10 months but it could be more."
The drug was approved for women in the US in February 2015.
From October 2012 to when Pfizer submitted its application for the drug to be licensed in August 2014, it met the Food and Drug Administration (FDA) several times to agree the basis for submission. The FDA indicated it would approve the drug in these meetings and designated it a breakthrough therapy in April 2013.
It then approved the drug quickly for use once Pfizer had submitted its application.
Pfizer applied last August for a licence to the European Medicines Agency (EMA) for Ibrance to be used across Europe. It expects approval in the EU by the end of 2016.
Dr Harold Burstein, spokesman for the American Society of Clinical Oncology (Asco), said the FDA tended "to move drugs into the marketplace more quickly" than the EMA.
He added: "They seem more willing to to act on dramatic results shown in small numbers of patients."
In the study, presented at the Asco conference in Chicago, 666 women were split into two groups. The first received Ibrance together with the standard aromatase inhibitor Femara (letrozole), with the other group receiving Femara alone.
Results showed women in the first group lived an average of 10 months longer without disease getting worse. No overall survival data has yet been gathered.
A Nice spokeswoman said: "Our approach to assessing the value of new cancer drugs has been reviewed on a regular basis since we were set up in 1999 and is already more generous than for other conditions.
"An appraisal for palbociclib for metastatic breast cancer is proposed, and if this goes ahead it won't be possible to pre-judge the outcome."