BP had been told weeks before the explosion on the Deepwater Horizon...that the cement slurry that would be used for the well was unstable, the US presidential commission investigating the disaster said tonight...Halliburton, the US company responsible for the cement that was supposed to seal the well and prevent the fatal blow-out, carried out two tests of the mixture in February. These indicated that the foam slurry was unstable. Data from one of the tests was emailed to BP on 8 March, the commission said...These results appear to contradict public assurances made by Halliburton after the disaster on 20 April that it had tested the cement before pumping it into the Macondo well and that the tests had indicated that the cement would be stable.