The breast cancer drug Herceptin must be funded by the NHS, the drugs watchdog has said.
The PCT said Herceptin may not be effective for all patients
The National Institute for health and Clinical Excellence made its decision after rejecting an appeal by a local primary care trust.
Newbury and Community PCT raised concerns over treatment length, cost effectiveness and potential risks.
Herceptin targets the HER2 protein which can fuel tumours. Around 20% of breast cancers are HER2 positive.
Primary care trusts now have three months to implement the NICE guidance, which comes after a high-profile campaign by patients and charities for Herceptin to be available on the NHS.
NICE has said breast cancer patients with HER2 positive cancers who had a moderate or high chance of their cancer returning should be given Herceptin, also known as trastuzumab, after they have had surgery, radiotherapy or chemotherapy.
They should then receive it at three-week intervals for a year or until the disease returns.
However, patients should have their heart function tested before they begin taking Herceptin. Around 10% of women will have a heart condition which could be exacerbated if they take the drug.
Women who are given Herceptin should also have their heart function tested regularly, NICE recommends.
NICE chief executive Andrew Dillon said: "Our assessment of Herceptin shows that it is clinically and cost effective for women with HER2 positive early breast cancer.
"The guidance has been issued rapidly, to ensure consistent use across the NHS and was produced in draft form just two weeks after the licence extension for Herceptin was granted."
It targets the HER2 protein, which can fuel the growth of breast tumours
Herceptin prevents this process happening
Around a fifth of breast cancers are HER2 positive
It is already licensed for use in women with advanced breast cancer - where the disease has spread within the breast or to another organ
Early stage breast cancer refers to the first occurrence of the disease
The cost for one year's treatment with Herceptin is around £19,500
NICE published its draft guidance on Herceptin in June.
Newbury and Community PCT said the guidance did not take into account research which found some women responded to the drug better than others.
It also highlighted studies which revealed some patients benefited from a shorter period of treatment than the recommended 12 months - meaning one year for all might not be cost-effective.
And it warned the longer term risks and benefits for the drug were still unknown.
A recent US study found just over a quarter of patients with advanced breast cancer using Herceptin developed some form damage to their hearts.
Health Secretary Patricia Hewitt welcomed the NICE guidance.
She said: "This is a very important decision for breast cancer patients.
"It removes any uncertainties that might have remained in the NHS about the clinical and cost effectiveness of this drug.
"This means that all eligible patients with early stage HER2-positive breast cancer can now receive Herceptin on the NHS and local primary care trusts must provide funding for this treatment within the next three months if they are not already doing so."
Dr Sarah Rawlings from Breakthrough Breast Cancer said: "We have at last come to the end of a long journey to ensure access to this vital drug for women who could benefit.
Christine Fogg, joint chief executive of Breast Cancer Care said: "The potential benefits this drug could have for patients with a moderate or high risk of a recurrence of breast cancer have been shown to outweigh its associated risks.
"The challenge now is to ensure that all newly diagnosed patients, wherever they live, can access the HER 2 test to establish if Herceptin could help them without delay."
But she stressed: "Every patient should ensure they discuss potential side-effects with their GP."
And Professor John Toy, medical director at Cancer Research UK, said: "Herceptin is still a relatively new treatment so clinical experience will need to be gained for some years yet before the drug's full effects are known.
"Herceptin doesn't seem to have many severe side effects but it can affect the heart so it isn't recommended for women with certain heart problems."