One molecule called CCS blocks a protein called Ras, which is defective in approximately 30% of all cancers.

The other, called CCZ, stimulates the body's own immune system to target and kill cancer cells.

It is hoped the research, carried out by Queensland Institute of Medical Research, could lead to new anti-cancer drugs.

The extract studied by the scientists, bromelain, is a rich source of enzymes and is widely used as a meat tenderiser, to clarify beer and tan leather hides.

The Queensland team discovered that the extract also had pharmacological properties and could activate specific immune cells while, simultaneously, blocking the immune function of other cells.

Lead researcher Dr Tracey Mynott said: "We suspected that different components of the crude mixture might be responsible for bromelain's biological effects.

"In searching for these components, we discovered the CCS and CCZ proteins and found that they could block growth of a broad range of tumour cells, including breast, lung, colon, ovarian and melanoma."

New action

Both CCS and CCZ are protease enzymes, more usually associated with breaking down proteins, as in the digestive process.

Dr Mynott said it was the first time this class of enzymes had been shown to have a specific effect on the immune system.

"The way CCS and CCZ work is different to any other drug in clinical use today.

"Therefore, CCS and CCZ will represent a totally new way of treating disease and potentially a whole new class of anti-cancer agent."

Dr Julie Sharp, at Cancer Research UK, said: "The origin of many anti-cancer drugs can be found in nature.

"However, it's early days for this research and the real test will be to see if the effects seen in the lab can be reproduced successfully in patients."