'Live' pictures of spreading breast cancer cells have highlighted a biological pathway that scientists hope will lead to new treatments.

Researchers tagged cancer cells in laboratory mice with a protein that glowed blue when the tumour-spreading process was active.

The cells were driven from the main tumour into the blood by a biological 'control switch' called transforming growth factor beta (TGFb).

Once in the blood stream, they could easily travel to other parts of the body. Cancer spread to organs such as the brain or liver, known as metastasis, is what kills most patients with the disease.

The research opens up new avenues for developing drugs that prevent or reduce metastasis.

Study leader Dr Erik Sahai, from the charity Cancer Research UK, said: "The results helped us to find the set of genes that are behind the spread of breast cancer - and that the genes need to be first turned on and then off in order for single cancer cells to be able to 'relocate'."

The findings were published in the journal Nature Cell Biology and also presented at the annual meeting of the American Society for Cell Biology in San Diego, California.

Previous research had shown that TGFb can regulate normal cell growth and movement.

In the new study, cells were observed as one-by-one they broke away and entered the blood circulation.

The blue tag protein showed when the TGFb control system was active. Metastasis occurred when TGFb first turned on certain messenger genes in the cancer cells and then turned them off.