The drug Herceptin cuts the risk of tumours returning in women with early stage breast cancer by 50%, a US study has shown.
The drug is already used for advanced cancer
A second study in the New England Journal of Medicine showed Herceptin can also reduce cancer recurrence when combined with a chemotherapy drug.
A leading cancer expert called the studies' findings "stunning".
But they will increase pressure on regulators to make Herceptin available for early as well as advanced cancers.
Herceptin, which also has the generic name traztuzumab, targets a protein called HER2, which appears to be over-abundant in some women's breast cancers.
An estimated fifth of breast cancer cases - around 10,000 women in the UK - are HER2 positive.
Roche, which makes Herceptin, has to submit an application to European regulators before the drug can be licensed for use in early-stage disease.
The National Institute for Health and Clinical Excellence then has to give its approval for the drug to be prescribed on the NHS.
The government has said this process should be fast-tracked. But it is likely to be next spring before Herceptin goes through all these regulatory hoops.
Until then, it will be up to primary care trusts (PCTs) to decide if they will fund the treatment for women, which can cost £30,000 a year.
On Tuesday, PCTs in Devon and Cornwall announced they would fund the treatment, and last month Barbara Clark won her battle to persuade Somerset PCT to pay for her to have the drug.
The first study, by the Breast Cancer Group of international researchers and sponsored by Roche, looked at the effect of giving Herceptin to breast cancer patients who had already had surgery and a course of chemotherapy.
Almost 1,700 women received one year's treatment with Herceptin and a group of equal size were simply observed.
After 12 months, 13% (220) of the group under observation had seen a recurrence of their cancer, 34 of whom had died.
In the group taking Herceptin, just 127 women (7.5%) had seen a recurrence of their cancer, with 23 cancer-related deaths.
Women on a two-year course of Herceptin are currently being followed up by the same researchers. Results so far are also promising.
Professor Ian Smith, head of the Royal Marsden Hospital and Institute of Cancer Research, who worked on the study, told the BBC News website: "This is one of the most important developments in breast cancer treatment.
"It is only a minority of women who will benefit from it, but for that minority it makes an enormous difference."
He said women should be given the drug before it has passed through the regulatory process.
"Normally, it's reasonable to wait until the drug is licensed.
"But these results are so unprecedented that it requires an unprecedented response, which is to make it accessible before it is licensed."
The second study combined results from two US trials covering 3,350 women given Herceptin either in combination with the drug paclitaxel (Taxol) or alone. Other women were given Taxol alone.
Taking both drugs together reduced the risk a woman's cancer would return by half, and the risk of death by a third.
Dr John Toy, medical director at Cancer Research UK, said: "These are stunning results for women who have got this particular sort of breast cancer.
"Each of the trials shows undoubted benefits."
He added: "These papers are in a top medical journal and the data is robust."
But Dr Toy said it would be important to see longer-term results from the studies, as cancers often recurred in HER2 positive women 18 months to two years after their initial diagnosis.
Dr Toy said he hoped there would be no unnecessary delay in the regulatory process for the drug to be approved for use in women with early-stage breast cancer.
He said that, until that time, PCTs would be faced with a decision over whether to give a drug for an unlicensed use to women who would be asking for it.
Jeremy Hughes, chief executive at Breakthrough Breast Cancer, said the studies showed one of the "biggest breakthroughs in breast cancer treatment."
Clara MacKay, of Breast Cancer Care, added: "Given this evidence, combined with mounting pressure from breast cancer patients and their families, the government must help ensure all can get equal access to Herceptin."
A Department of Health spokesperson said no decision on using Herceptin in the NHS could be made until the makers applied for a license to extend its use.
She added: "We must be sure that medicines are robustly tested and proved effective before they are made available to patients.
"If and when it is licensed for early use, NHS bodies can prescribe Herceptin, but this must be a local decision."
But Shadow Health Secretary Andrew Lansley said: "It remains to be seen how PCTs will pay for the treatment.
"The NHS is already crippled by worsening debts, which entrench the postcode lottery."