NGT kept ownership of four other distribution networks, which make up almost half of Great Britain's gas distribution network. In August 2004, National Grid Transco agreed to sell four of its regional gas distribution networks for a total cash consideration of £5.8 billion. In 2004, the company was found liable for a gas explosion in Transco plc v HM Advocate and subsequently fined £15 million. which was acquired as part of the merger with Lattice Group for a nominal £1 to Hutchison Whampoa in December 2002. National Grid Group changed its name to National Grid Transco plc. In October 2002, National Grid Group merged with Lattice Group, owner of the Transco gas distribution business (Lattice had demerged from BG Group in 2000). In January 2002, National Grid Group acquired Niagara Mohawk Power Corporation, a New York State utility. In March 2000, National Grid Group acquired United States companies New England Electric System and Eastern Utilities Associates.

With the beginning of the new millennium, National Grid pursued mergers and international acquisitions.

The company was first listed on the London Stock Exchange in December 1995. In 1990, the transmission activities of the CEGB were transferred to the National Grid Company plc, which was owned by the twelve regional electricity companies (RECs) through a holding company, National Grid Group plc. National Grid and acquisitions (1990–1999) Its generation (or upstream) activities were transferred to three generating companies - PowerGen, National Power, and Nuclear Electric (later British Energy, eventually EDF Energy)-and its transmission (or downstream) activities to the National Grid Company. The present electricity market in the United Kingdom was built upon the breakup of the CEGB into four separate companies in the 1990s. Main article: Central Electricity Generating Boardīefore 1990, both the generation and transmission activities in England and Wales were under the responsibility of the Central Electricity Generating Board (CEGB).

National grid gas account types