A method of operation of a cellular wireless communications network, the method comprising, in respect of a cell of the cellular wireless communications network: receiving measurements from a plurality of user equipments relating to the reference signal received powers from a plurality of other cells of the cellular wireless communications network; for a plurality of said other cells of the cellular wireless communications network having a physical cell identity that is the same as a physical cell identity of said cell, determining based on said received measurements whether a mutual potential interference level with said cell exceeds a threshold; and in the event that the mutual potential interference level between the cell and a potentially interfering cell exceeds a threshold: for the cell, generating a first list of physical cell identities available in the cellular wireless communications network; for the potentially interfering cell, generating a second list of physical cell identities available in the cellular wireless communications network; identifying an alternative physical cell identity from said first and second lists that would cause a lowest mutual potential interference level of a) interference between the cell and another cell already using that alternative physical cell identity, and b) interference between the potentially interfering cell and said another cell, and if the lowest mutual potential interference level is lower than the mutual potential interference level between the cell and the potentially interfering cell, selecting said alternative physical cell identity as a new physical cell identity for the cell or the potentially interfering cell. receiving measurements from a plurality of user equipments relating to the reference signal received powers from a plurality of other cells of the cellular wireless communications network; for a plurality of said other cells of the cellular wireless communications network having a physical cell identity that is the same as a physical cell identity of said cell, determining based on said received measurements whether a mutual potential interference level with said cell exceeds a threshold; and in the event that the mutual potential interference level between the cell and a potentially interfering cell exceeds a threshold: for the cell, generating a first list of physical cell identities available in the cellular wireless communications network; for the potentially interfering cell, generating a second list of physical cell identities available in the cellular wireless communications network; identifying an alternative physical cell identity from said first and second lists that would cause a lowest mutual potential interference level of a) interference between the cell and another cell already using that alternative physical cell identity, and b) interference between the potentially interfering cell and said another cell, and if the lowest mutual potential interference level is lower than the mutual potential interference level between the cell and the potentially interfering cell, selecting said alternative physical cell identity as a new physical cell identity for the cell or the potentially interfering cell. for the cell, generating a first list of physical cell identities available in the cellular wireless communications network; for the potentially interfering cell, generating a second list of physical cell identities available in the cellular wireless communications network; identifying an alternative physical cell identity from said first and second lists that would cause a lowest mutual potential interference level of a) interference between the cell and another cell already using that alternative physical cell identity, and b) interference between the potentially interfering cell and said another cell, and a) interference between the cell and another cell already using that alternative physical cell identity, and b) interference between the potentially interfering cell and said another cell, and if the lowest mutual potential interference level is lower than the mutual potential interference level between the cell and the potentially interfering cell, selecting said alternative physical cell identity as a new physical cell identity for the cell or the potentially interfering cell.