Doctors and cancer charities have been asked to help trace 5,000 women who are at high risk of breast cancer after receiving radiotherapy treatment for the immune system disorder Hodgkin's disease.

The Department of Health said yesterday that research showed up to one in three women given the treatment in childhood, and up to one in four who received it as a young adult, had developed breast cancer within 25 years. The average risk of developing breast cancer by 50 is one in 50.

The treatment, called supradiaphragmatic radiotherapy, involves irradiating the neck, breast area and armpits of affected women and has been used since 1962. Women who were under 35 at the time of treatment are at greatest risk but are not invited for breast screening until the age of 50.

Professor Mike Richards, the Government's cancer tsar, said the radiotherapy was life-saving and tracing the women was not prompted by any mistakes in treatment. He said: "Radiotherapy is very effective in patients with Hodgkin's disease. Without radiotherapy many of these patients would have died. However, we now know that more of these patients are developing breast cancer than would normally be expected. That is why we are taking action to alert patients and to offer them screening to try and pick up any cancers early."

Evidence of the harmful effects of the radiotherapy only came to light in May and an expert advisory group was set up. Women who received the treatment will be offered annual breast screening and cancer charities are to assist in tracing as many of the women as possible by January.

Professor Richards said the type of radiotherapy that was causing concern was less common now in the treatment of Hodgkin's, which is a cancer of the immune system.

Chemotherapy on its own or in conjunction with more targeted radiotherapy has been used more frequently since the 1980s and is not linked with an increased risk of breast cancer.

Supradiaphragmatic radiotherapy has also been linked with a slightly higher risk of lung and stomach cancer.

The women who can be contacted through local cancer centres will be sent a letter inviting them for a consultation. It is hoped other women not on records will come forward.

The charity CancerBACUP is one of the groups backing efforts to spread the message. Joanne Rule, chief executive of CancerBACUP, said: "It is absolutely right that women in this group are contacted. In some cases women will need extra screening and in all cases they will need information and support to help them cope with the implications of this new evidence."

Women concerned they may be at risk should call the dedicated helpline on 0845 850 9850.