In its first-ever ruling on a case involving global warming, the court decided that the Environmental Protection Agency (EPA), a White House department, was authorised to enforce limits on the amount of carbon dioxide produced by vehicles. It said the EPA had offered "no reasoned explanation" for its refusal to regulate such emissions and Justice John Paul Stevens said the EPA's claim that it lacked the authority was "arbitrary, capricious or otherwise not in accordance with law".

Under President George Bush, the White House has long refused to enforce binding limits on greenhouse gas emissions, claiming such a move would damage the economy. It has argued the way forward for the US - responsible for 25 per cent of global emissions and 37 per cent of the world's vehicles - is to engage in voluntary reduction through the use of more efficient technology.

But the government's position is increasingly out of touch with a number of US states and individual cities which have agreed to enforce reductions. Last year California introduced legislation to cut emissions by 25 per cent over the next 12 years. Other states - some of which were behind the lawsuit that the Supreme Court ruled on - have established a carbon trading network. But the EPA had argued that even if it had the legal power to enforce regulations, it was not obliged to do so.

The lawsuit had asked the court to consider the EPA's obligations under the 1970 Clean Air Act. This legislation requires the EPA to protect the public from exposure to airborne contaminants.