David Cameron signalled his support for a gas-fired future yesterday when he threw his weight behind a ?13 billion supply agreement with Norway.  amid mounting concern about the cost of renewables and nuclear delays, the Prime Minister predicted that gas would play a "central role" for decades as he backed a big gas deal between Centrica and Statoil.  Centrica, the company behind British Gas, will purchase 50 billion cubic metres of gas from Statoil in a ten-year deal starting from 2015, enough to supply 3.5 million homes. That is equivalent to 5 per cent of Britain's total annual gas demand, Centrica said.  Centrica will also purchase ?1 billion of oil and gas-producing assets from the state-controlled Norwegian group and will sign up to a gas exploration partnership.  The deal will tie in a chunk of Norwegian gas supplies as Britain's domestic North Sea reserves dwindle and competition for stable gas supplies grows across Europe. Gas powers half of Britain's electricity and nearly 70 per cent of its heating. The deal will allow Statoil to maintain its 16 to 18 per cent market share in Britain.  David Hunter, an analyst at M&C Energy Group, said that Britain was likely to embark on a "second dash for gas" in the second half of the decade as old coal-fired capacity is retired and doubts hang over new nuclear production and renewables.  "There is a big concern over energy security, and a lot of unanswered questions on the extent of the role for renewables and role that nuclear can play," he said.  "From the point of view of energy security, obtaining sources of longterm supply is important," he said. Taking it from our near-neighbour Norway made "good strategic sense".  Centrica will also