Issue group allocation circuitry controls allocation of each micro-operation to one of a plurality of issue groups, depending on detection of register conflicts between micro-operations, the register conflicts concerning access to registers of a first register set. A given micro-operation is allocated to a selected issue group for which no micro-operation already allocated to the selected issue group has a register conflict with the given micro-operation and the selected issue group is a younger issue group than any issue group already allocated an older micro-operation than the given micro-operation for which a register conflict is detected between the given micro-operation and the older micro-operation. Issue circuitry controls issue of the micro-operations based on the issue groups, to prevent any instruction in a given issue group being issued until all micro-operations in any older issue group than the given issue group have been issued.