Before execution of a routing area update (RAU) procedure associated with a handover of a mobile station (40) from a source cell to a target cell in a mobile telecommunication system, a handover required message (14) is sent to a core network (CN) from a source base station (16) in the source cell having a mobile station identifier (IMSI), an identifier of at least one target cell routing area code (RAC) and a temporary logical link identity (TLLI). In response thereto, a handover command message (32) is sent to the source base station from the core network having a packet-temporary mobile subscriber identity (P-TMSI) for transfer to the mobile station in ciphered form over a radio interface between the source base station and the mobile station so that the mobile station is able to send packet-switched (PS) data on an uplink before the RAU procedure is executed.