A high pressure pump assembly for a common rail system comprises a casting, a feed pump mounted to the casting for drawing fuel from a fuel reservoir and pre-pressurizing the fuel, a high pressure pumping unit mounted in the casting for receiving the pre-pressurized fuel from the feed pump, further pressurizing the fuel and supplying it to a rail of the common rail system, a shared driving means for operatively driving both the feed pump and the high pressure pumping unit, and a flow regulator for controlling the fuel pressure in the rail and the fuel pumping procedure.