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This paper describes the data communications activity at the National Physical Laboratory (NPL) during the period 1965-1975. The key feature of the NPL work was the development of the principle of packet switching, which was first proposed in a data communications context by D. W. Davies of the NPL in 1965. The report focuses on the construction of the NPL Data Communications Network, which first became operational in 1970. This network served both as a model for a possible U.K. national network and as a practical local area network (LAN) for the NPL site. The report describes the impact of the NPL work on other early networks, such as ARPANET and the British Experimental Packet-Switched Service (EPSS), and on data communications in general.