a British government decision to underwrite a billion-dollar loan to one of the world's biggest oil companies came under fire from environmentalists last night. Groups including WWF and Greenpeace accused ministers of reneging on a pledge not to support investment in "dirty fossil fuel".  The deal, which will see UK Export Finance guarantee a loan to the Brazilian oil firm Petrobras for deep-water oil drilling, is contrary to a government commitment that the department, which is headed by Vince Cable, the Business Secretary, and the trade minister, Lord Green, would champion British firms involved in green technologies "instead of supporting investment in dirty fossil-fuel energy production".  Joss Garman, a Greenpeace energy campaigner, said: "Ministers explicitly ruled out using taxpayer's money to support dirty fossil-fuel projects in their coalition agreement. So it's brazen hypocrisy for them to be risking hundreds of millions of pounds of our money backing dangerous deep-sea oil drilling off Brazil's coastline when they should be supporting clean energy projects. It's yet another broken promise."  Petrobras was involved in 18 major oil and gas spills between 1975 and 2001, in which 141 people were killed and about 29 million barrels of oil spilled, according to Greenpeace Brazil. In 2001, the world's larg