An integrated circuit is provided with interface circuitry used to provide an interface between functional circuitry of the integrated circuit and components external to the integrated circuit. The interface circuitry includes a plurality of interface cells having interface components configured to operate from a first power supply. Each interface cell incorporates a power supply line section extending across its width and configured to cooperate with power supply line sections of other interface cells to provide a power supply line structure shared by the plurality of interface cells, for provision of the first power supply to the interface components. Each power supply line section includes a first supply line portion and a second supply line portion, the first supply line portion being sized to support a current carrying constraint of the interface circuitry, while the second supply line portion is sized insufficiently to support that current carrying constraint. Instead, each interface cell is arranged within the interface circuitry such that the second supply line portion is connected to a current carrying structure external to that interface cell, such that the second supply line portion in combination with the connected current carrying structure is sized sufficiently to support the current carrying constraint. Such an approach enables a reduction in the area required for the power supply line structure, hence enabling a reduction in the size of the interface circuitry, and accordingly a corresponding reduction in the required size of the integrated circuit.