A new class of breast cancer drugs is to receive preliminary approval for use in the NHS today, raising the prospect of improved quality of life for some women, but at a substantially increased cost.

The drugs, known as the aromatase inhibitors, will be approved for use alongside tamoxifen, for decades the "gold standard" treatment for post-menopausal women who have undergone surgery for a first breast cancer. Although doctors will have the option to prescribe either the new drugs or tamoxifen, most experts believe the aromatase inhibitors will gradually replace the standard drug.

But the three aromatase inhibitors that the National Institute for Clinical Excellence (Nice) will tentatively approve in draft guidance today - anastrozole, exemestane and letrozole - cost about £1,000 per patient per year, 10 times more than tamoxifen. About 20,000 women have the type of cancer that is susceptible to the drugs, which suggests that the NHS drugs bill could rise significantly.

In its draft guidance, which will be published on the Nice website for consultation, the institute says the trials of the new drugs have shown fewer women on them have a recurrence of cancer than on tamoxifen. In the largest study of anastrozole, after five years, 81.3% of women on the drug were alive and disease-free compared with 79% of those on tamoxifen. In a study of exemestane, after three years 89% were alive and cancer-free compared with 85.1% of those on tamoxifen.

After two years, the figures for letrozole were 91.2% compared with 89.3% on tamoxifen. But, the Nice guidance says, there is no evidence of an improvement in overall survival - in the long-term, it is possible that the same numbers of women will die of their cancer.

The side-effects of the drugs are different. Women on aromatase inhibitors have more bone fractures, while women on tamoxifen have more endometrial cancers. Nice says doctors must decide which drugs are most suitable for the patient.

Breast cancer charities, which have campaigned strongly for newer drugs, welcomed the draft guidance. "Today's initial decision is a welcome one for thousands of women in Britain and will hopefully lead to an alternative early breast cancer treatment," says Joanne Rule, chief executive of Cancerbackup. She said there was nothing to stop primary care trusts agreeing to pay for a new drug before Nice issued its final guidance, which is expected in November. Unlike Herceptin, these drugs are licensed.

Jeremy Hughes, chief executive of Breakthrough Breast Cancer, said: "Tamoxifen has already had a big impact on women's lives and these new treatment options will be an important addition to the armoury of therapies available to treat women with the disease."