LONDON, March 27 (Reuters) - Two international medical humanitarian organisations said on Monday they had challenged the patent on Gilead Sciences' hepatitis C drug sofosbuvir at the European Patent Office in order to increase access to the treatment.

Sofosbuvir, sold by the U.S. drugmaker as Sovaldi, is transforming the liver-destroying viral disease by offering an effective cure, but Medecins Sans Frontieres (MSF) and Medecins du Monde (MdM) said its high cost was a serious barrier.

The campaigners argue that the patent on the drug is open to challenge because the science behind sofosbuvir is not new.

If the patent challenge is successful, it could make cheaper generic versions of sofosbuvir available in Europe. MSF and MdM said key patents on sofosbuvir had already been revoked in China and Ukraine, and decisions were pending in other countries.

Officials at Gilead were not immediately available to comment.