BP and Halliburton are at greater risk of a substantial pay-out over the Gulf of Mexico oil spill after a US investigation claimed they knew cement used to seal the well was probably unstable, legal experts said on Friday. One high-profile lawyer for individuals suing BP proclaimed the evidence a “game-changer”, while independent observers said it raised the chances of a gross negligence finding against the companies that could trigger penalties and court awards running into billions of dollars...Doug Kysar, professor at Yale Law School, said the report was “extremely damaging” both to Halliburton as the cement tester and to BP...Legal experts said the report was also ominous for the corporate defendants because it undermined their ability to present a united front against lawsuits and instead increased the likelihood they would attack each other and thus help plaintiffs.