Breast milk is being used to fight cancer after scientists accidentally discovered it contains a substance that kills tumour cells.

Trials in patients with bladder cancer have already yielded promising results and researchers believe the compound breast milk contains – nicknamed Hamlet – will also help tackle bowel cancer and cervical cancer.

They also say it homes in on cancer cells while leaving healthy cells unharmed – so it has none of the debilitating side effects of chemotherapy.

Professor Catharina Svanborg, who made the initial discovery, said last night: 'There's something magical about Hamlet's ability to target tumour cells and kill them.'

She said human breast milk contained a protein called alpha-lactalbumin, which is transformed into a cancer-fighting agent when in the gut.

Prof Svanborg, an immunologist at Lund University in Sweden, made the chance discovery that the substance kills tumour cells when working on antibiotics.