A data transmission method of a User Equipment, UE, in a Long Term Evolution, LTE, compliant mobile communications network, and a corresponding UE. The method comprises detecting reconfiguration of a bearer from a split bearer in which uplink Packet Data Convergence Protocol, PDCP, Protocol Data Units, PDUs, are transmitted to both a Master eNB, MeNB, and to a Secondary eNB, SeNB, to a non-split bearer in which uplink PDCP PDUs are transmitted only to the MeNB. If reconfiguration of a bearer from a split bearer to a non-split bearer in which uplink PDCP PDUs are transmitted to the MeNB is detected, the method further comprises initiating retransmission of PDCP PDUs from the first PDCP PDU for which transmission was attempted via the SeNB and for which there has been no confirmation of successful delivery by a protocol layer below the PDCP layer within the UE. The method further comprises retransmitting only PDCP PDUs for which transmission of the PDU was attempted via the SeNB.