The present application relates to process for producing a glass fibre-reinforced thermoplastic polymer composition, which comprises the sequential steps of: a) unwinding from a package of at least one continuous glass multifilament strand, b) applying a sheath of thermoplastic polymer around the at least one continuous glass multifilament strand to form a sheathed continuous multifilament strand; The unwound, the at least one continuous glass multifilament strand is transported through the inside of at least one protective tube over a certain distance between the location of unwinding from a package of said at least one continuous glass multifilament strand, i.e. step a), and the location of applying said sheath of thermoplastic polymer around said at least one continuous glass multifilament strand, i.e. step b).