The present invention relates to a flavonoid compound having the formula I, where R3, R5 and/or R7 is a group having the formula II, or R1 and R2 are both OH groups, or to one of the pharmaceutically acceptable salts or esters thereof, for use as an antiviral agent in the treatment and/or prevention of a hepatitis C virus (HCV) infection. The invention also relates to an ex vivo method for reducing the infectivity of HCV or for inactivating HCV, including a step of contacting said hepatitis C virus with a compound having the formula (I).