Viruses do not cause breast cancer or a common type of brain tumour, scientists have discovered.

According to a team from Sweden, it has already been proven that around 15% of all cases of cancer arise as a result of a viral infection, however some believe this figure is actually much higher.

For example, some researchers believe that the Epstein-Barr virus - a common virus belonging to the herpes family - can cause breast cancer.

The scientists decided to look into this further, focusing on breast cancer and the brain tumour, glioblastoma.

As part of their study, they used powerful computers to carry out an in-depth analysis of over seven billion DNA sequences from more than 4,000 tumours.

They found no links between these cancers and any viruses.

"We conducted an unbiased search for viruses in 19 different types of cancer. The analysis has neatly confirmed the links that are already known, which effectively confirms that we are using the right method.

"If there were any viral involvement in breast cancer and glioblastoma, it is likely that we would have found some trace of it. We have based our research on American material, which is extremely comprehensive," the scientists from the University of Gothenburg said.

However, they were able to confirm that liver cancer is often caused by the hepatitis virus and that the human papilloma virus (HPV) causes cervical cancer.

Details of these findings are published in the journal, Nature Communications.