1. A method of encoding a sequence of digital images into a bitstream, at least one portion of an image being encoded with respect to a reference image portion by a motion estimation process that determines motion information identifying the reference image portion, the method comprising encoding said motion information using a motion information prediction process comprising: obtaining, for an image portion of an image to be encoded, if available, one or more actual motion information predictors having motion vectors obtained from motion vectors associated with first image portions of said image to be encoded; if a number of the one or more obtained motion information predictors is lower than a target number, wherein the target number is greater than one, adding one or more motion information predictors to generate a set of motion information predictors until a number of motion information predictors in the set is equal to the target number, wherein the adding of one or more motion information predictors comprises: performing a first process of adding, if available, one or more further actual motion information predictors having motion vectors obtained from motion vectors associated with second image portions of said image to be encoded or of the different image from said image to be encoded; and if a number of motion information predictors after the first process is lower than the target number, performing a second process of adding one or more virtual motion information predictors, the one or more virtual motion information predictors having motion vectors which are neither associated with image portions of said image to be encoded nor of the different image from said image to be encoded; and selecting a motion information predictor for encoding motion information of said image portion of said image to be encoded from the generated set of motion information predictors.