A method of operating a second network access node in a wireless telecommunication system comprising a terminal device, a first network access node and the second network access node, wherein the method comprises: configuring the second network access node to act as a secondary network access node for a dual connectivity mode of operation for the terminal device in which the first network access node acts as a master network access node, wherein the first network access node is associated with a master network access node security key and the second network access node is associated with a secondary network access node security key, wherein the secondary network access node security key is derived from the master network access node security key and is established by the second network access node from information received from the first network access node; establishing, while the second network access node is acting as secondary network access node for the dual connectivity mode of operation for the terminal device, that the second network access node should switch to acting as a master network access node for the terminal device; deriving a new master network access node security key to be used by the second network access node when it is switched to acting as a master network access node for the terminal device; and configuring the second network access node to act as a master network access node for the terminal device using the new master network access node security key characterized by wherein the new master network access node security key is derived from the secondary network access node security key by combining the secondary network access node security key with a predetermined number. configuring the second network access node to act as a secondary network access node for a dual connectivity mode of operation for the terminal device in which the first network access node acts as a master network access node, wherein the first network access node is associated with a master network access node security key and the second network access node is associated with a secondary network access node security key, wherein the secondary network access node security key is derived from the master network access node security key and is established by the second network access node from information received from the first network access node; establishing, while the second network access node is acting as secondary network access node for the dual connectivity mode of operation for the terminal device, that the second network access node should switch to acting as a master network access node for the terminal device; deriving a new master network access node security key to be used by the second network access node when it is switched to acting as a master network access node for the terminal device; and configuring the second network access node to act as a master network access node for the terminal device using the new master network access node security key characterized by wherein the new master network access node security key is derived from the secondary network access node security key by combining the secondary network access node security key with a predetermined number.