Scientists have claimed that up to 40 per cent of cancers are caused by viruses, a finding which could soon pave the way for new vaccines against the diseases and therapies to cure them.

A team at the Karolinska Institute in Sweden says it has found a viral link with medulloblastoma, the most common form of childhood brain tumour.

It follows the discovery two years ago that Merkel cell carcinoma, an aggressive skin cancer, often follows infection by the polyomavirus which is common among animals and can spread to humans.

Moreover, the hepatitis B and C bugs have been often found to cause liver cancer, and the human papilloma virus is linked to cervical cancer. It's claimed that prostate cancers could be caused by viruses too, the 'Daily Mail' reported.

Nobel Prize winner Harald zur Hausen, who jointly discovered the link between cervical cancer and HPV in the 1980s, said that he expected more discoveries to follow and suggested that viruses could be involved in cancer of the skin, breast, gut and lungs.

But scientists warn it could take a long time and huge investment before vaccines are developed.

Alan Rickinson, professor of cancer studies at Birmingham University, said: "If we can understand how these viruses work we could prevent people from contracting them and even create therapies that use the patient's own immune system to destroy infected or cancerous cells."

The process still confounds the experts as viruses work by invading cells and making them produce more viruses. But this process then kills the cell which should mean it cannot become cancerous.

One theory is that cancer-causing viruses can remain hidden in cells for years, preventing the cell from repairing mutations.