Nicolas Sarkozy chose a thermal technology factory in eastern France to unveil the details of his carbon tax plan. Starting next year, France will levy a tax on CO2 emissions from oil, gas and coal, set at 17 euros per tonne. Sarkozy said this reflected CO2's market value.

According to the president, that would work out at around 4 cents extra for petrol, a bit more for diesel and heating oil, and an increased gas bill of around 50 euros a year for the average French household. Most electricity in France comes from nuclear power or renewables, and will be exempted. Heavy polluters are also exempted, since they are part of the EU's emissions trading scheme. The aim of course, is to incentivise people to pollute less.

At today's levels of fuel consumtion, the tax would bring at least three billion euros into state coffers. But Sarkozy insisted tax cuts elsewhere would ensure no one ends up out of pocket.

Perhaps a little sceptical of these promises, more than two thirds of French people say they oppose the tax. Environmental groups, meanwhile, say it is set too low to have any impact.

The idea of a carbon tax is not a new one. Sweden set a levy of 27 euros per tonne nearly twenty years ago. It's since risen to 108 euros. And though farmers complain it has hurt them, other firms say it encouraged them to go green. Four other European countries have similar scheme to the one Sarkozy proposes, while in Canada, the state of British Columbia has a carbon tax, albeit at under 10 euros a tonne.

As the largest country to introduce a carbon tax, France hopes to lead by example at December's climate change conference in Copenhagen. Five foreign ministers met there this Tursday to prepare for the meeting - insisting again on the need for drastic emissions reductions. But as the fierce debate over carbon tax in France shows, every step is a political battle.