An early form of breast cancer has risen by 30 per cent in five years, researchers reported yesterday.

The rise in the cancer, called ductal carcinoma in situ (DCIS), is attributed to the success of the breast screening programme which is detecting tumours that would otherwise be missed.

But the development has divided specialists over the best way of treating the condition. In 60 per cent of cases, DCIS resolves naturally and does not develop into an invasive cancer.

A total of 3,766 women were diagnosed with DCIS in 2002, an increase of 856 compared with 1998, according to figures published by Cancer Research UK.

The condition occurs when cells inside the milk ducts in the breast change into cancerous cells, forming a lesion about the size of a pencil point.

Although most do not spread, it is difficult to tell which, so all are investigated causing anxiety and pain to the woman.

The new figures were announced at the start of a worldwide study to investigate which of two breast cancer drugs, the old treatment tamoxifen or the new treatment, anastrozole, is better at preventing the disease from returning in women who have already had DCIS.

The Ibis-2 study aims to recruit 10,000 women, 4,000 of whom have been diagnosed with DCIS.

Concern about the rise in cases of DCIS has led some doctors to criticise the screening programme for increasing anxiety and subjecting women to unnecessary treatment without extending their lives.

A study published in the British Medical Journal in 2003 said breast screening was contributing to a rise in the incidence of breast cancer and resulted in mastectomies that may have been unnecessary for the women and expensive for the health service.

Professor Jack Cuzick of Cancer Research UK said the rise in cases of DCIS could be seen as either a gain or a loss for women.

"That is a key issue. DCIS is quite potentially over-treated. It is important to tailor treatment for those affected."

Professor Cuzick said thatthe questions were whether to perform a mastectomy (removal of the whole breast) or lumpectomy (removal of the lump from the breast), whether to follow surgery with radiotherapy, and whether to give hormonal treatment with the drugs which are the subject of the Ibis trial.

The NHS breast screening programme began in 1988. Professor Cuzick said: "With time, radiologists are getting better and better at identifying DCIS. About 30 per cent of cases will progress over five years. For women that is probably something that is worth knowing about."

The breast screening programme is funding a study called the Sloane project to collect information on cases of DCIS to establish the best form of treatment. The study began in 2003 and is due to complete data collection in March 2008.