The deal, which follows a battle with BP and Statoil for the rights, will also provide British Gas with a fresh flow of natural gas into western Europe.
The Karachaganak field has gas reserves of around 20,000 billion cubic feet, four times the size of the Morecambe field in the North Sea and enough to meet Britain's needs for a decade. The field also has two billion barrels of oil, equivalent to the UK Forties field.
British Gas regards Kazakhstan as vital in achieving its aim of earning 40 per cent of total revenues from exploration and production by the end of the decade.
The field is described as a 'supergiant' in world terms. It will take advantage of the pipeline infrastructure established for the neighbouring Orenburg field in Russia, now in decline.
British Gas said that its initial commitment was dollars 20m, rising to dollars 3bn over a decade, although this should be offset by increasing production levels. Agip will bring the total investment to almost dollars 6bn.
The field has been in production since 1984 but at a very low level. Agip said that the existing production eliminated some of the risks associated with exploration and production projects.
The government of Kazakhstan will operate a profit-sharing arrangement with the companies, which could mean taking either gas for domestic consumption or cash.
Howard Dalton, managing director of British Gas exploration and production, said: 'Karachaganak represents substantial progress towards achieving our stated aims of creating a major international energy business.'
The company is increasingly keen to expand beyond its regulated business in Britain, where it faces a sharp reduction in its industrial market and the erosion of its monopoly over domestic gas sales.
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