Bowel cancer patients in Wales will be the first in the UK to be treated on the NHS with a drug called cetuximab.
The drug is marketed under the name Erbitux
Welsh Health Minister Brian Gibbons has approved the drug - marketed as Erbitux - which can delay the spread of advanced cancer and shrink tumours.
Campaigners say it means patients in Wales will get better treatment than those in other parts of the UK.
But experts and the assembly government expect approval to be overturned in November by the UK's health watchdog.
The National Institute for Health and Clinical Excellence (NICE) is due to confirm a negative appraisal when it reports back in November.
Mid and west Wales Conservative AM Glyn Davies, a recovered bowel cancer patient, welcomed the ratification of the drug, but said there would be "major concern" in England that it would not be available over the border.
"I hope that by next year bowel cancer sufferers in England will have the same access to this life saving drug as we now have in Wales," said Mr Davies.
The director of the Wales Cancer Trials Network, Professor Tim Maughan welcomed the approval as "excellent news" for cancer patients.
BOWEL CANCER FACTS
Bowel cancer is the UK's second biggest cancer killer
35,000 are diagnosed every year - 95 a day
16,000 die from bowel cancer every year - 44 a day
30% of patients do not present themselves until the cancer has spread, making treatment more difficult
The five-year survival rate for patients with advanced bowel cancer is less than 5%
Prof Maughan, consultant oncologist at Cardiff's Velindre Hospital, said it was not a "magic wonder drug" and was also "horrifically expensive," but patients who would benefit from it were "desperate" because all other forms of cancer therapy would have failed.
Suitable patients will receive 18 weeks of treatment on the NHS costing £600 a week.
"It provides a short extension to life for some of the patients who receive it. For more than half of the patients who receive it does nothing," said Prof Maughan.
He is conducting a UK-wide trial into the success of the drug which is due to finish in 18 months.
He hoped the trial would put pressure on the health watchdog to approve cetuximab by helping define the patient group for whom it really worked.
He estimated about 25 patients a year in Wales would benefit from the drug, and although its approval is expected to be reversed later in the year, he said it would "be crazy to say we won't do it".
An assembly government spokesman said NICE was recommending in draft guidelines that cetuximab - licensed for use in the UK two years ago - should not be used.
"Once that comes into place, if it stays the same, it would override what we have already decided. But it is available for now," said the spokesman.
Bowel Cancer UK welcomed the decision, which was taken after approval in March by the All-Wales Medicine Strategy Group (AWMSG).
But its chief executive Neil Brookes said it also highlighted "the inequity of access to bowel cancer treatments across the UK".
"Patients in Wales are now able to receive treatments that those in England and Scotland cannot," he said.
"We call on NICE and the Scottish Medicines Consortium to follow Wales's lead and approve these drugs for use in England and Scotland, thus ending the postcode - or even country - lottery surrounding bowel cancer treatments in the UK."
He added: "We also fear that if NICE does not approve the use of Erbitux in November, Welsh patients will after all, be denied access - on the grounds of cost - to a treatment that has been recommended to them as clinically effective."