A method of operating a wireless communication terminal that is capable of communicating with a base station over a primary carrier and at least one secondary carrier, the method comprising: receiving, at said wireless terminal, one or more downlink control messages transmitted by said base station, wherein each downlink control message contains scheduling information scheduling the wireless terminal to receive a downlink transmission on either a primary carrier or a secondary carrier during a first subframe; determining, for each of the one or more downlink control messages, whether that downlink control message includes scheduling information scheduling the wireless terminal to receive a downlink transmission on the primary carrier or a downlink transmission on a secondary carrier; the wireless terminal selecting (708, 710) a format for a single uplink control message based on whether any of the one or more downlink control messages includes scheduling information scheduling the wireless terminal to receive a downlink transmission on a secondary carrier, wherein the format is selected from: a carrier aggregation format that includes one or more feedback bits for each of the configured carriers; and a single carrier format that includes feedback information for the primary carrier but does not include separate feedback bits for the at least one secondary carrier; the wireless terminal generating said uplink control message based on the selected format, wherein said uplink control message includes feedback information associated with at least one carrier, the feedback information indicating whether the wireless terminal has received a downlink transmission scheduled for the associated carrier; characterised in that: if (a) the carrier aggregation format is selected and (b) no scheduling information is included in the one or more downlink control messages for one or more of the configured carriers, the wireless terminal is configured to set feedback bits associated with the configured carriers for which no scheduling information was received to a fixed value; and transmitting the uplink control message to the base station. receiving, at said wireless terminal, one or more downlink control messages transmitted by said base station, wherein each downlink control message contains scheduling information scheduling the wireless terminal to receive a downlink transmission on either a primary carrier or a secondary carrier during a first subframe; determining, for each of the one or more downlink control messages, whether that downlink control message includes scheduling information scheduling the wireless terminal to receive a downlink transmission on the primary carrier or a downlink transmission on a secondary carrier; the wireless terminal selecting (708, 710) a format for a single uplink control message based on whether any of the one or more downlink control messages includes scheduling information scheduling the wireless terminal to receive a downlink transmission on a secondary carrier, wherein the format is selected from: a carrier aggregation format that includes one or more feedback bits for each of the configured carriers; and a single carrier format that includes feedback information for the primary carrier but does not include separate feedback bits for the at least one secondary carrier; the wireless terminal generating said uplink control message based on the selected format, wherein said uplink control message includes feedback information associated with at least one carrier, the feedback information indicating whether the wireless terminal has received a downlink transmission scheduled for the associated carrier; characterised in that: if (a) the carrier aggregation format is selected and (b) no scheduling information is included in the one or more downlink control messages for one or more of the configured carriers, the wireless terminal is configured to set feedback bits associated with the configured carriers for which no scheduling information was received to a fixed value; and transmitting the uplink control message to the base station. a carrier aggregation format that includes one or more feedback bits for each of the configured carriers; and a single carrier format that includes feedback information for the primary carrier but does not include separate feedback bits for the at least one secondary carrier; the wireless terminal generating said uplink control message based on the selected format, wherein said uplink control message includes feedback information associated with at least one carrier, the feedback information indicating whether the wireless terminal has received a downlink transmission scheduled for the associated carrier; characterised in that: if (a) the carrier aggregation format is selected and (b) no scheduling information is included in the one or more downlink control messages for one or more of the configured carriers, the wireless terminal is configured to set feedback bits associated with the configured carriers for which no scheduling information was received to a fixed value; and transmitting the uplink control message to the base station.