European leaders remained divided over whether or not to slap a carbon tax at EU borders at a two-day EU summit that wound up Friday.

French President Nicolas Sarkozy urged European Union leaders to agree to such a tax following the collapse of the Copenhagen climate summit.

But EU sources said some of the 27 heads of state and government remained sceptical, with Austrian Chancellor Werner Faymann notably arguing that "it wouldn't be a good negotiating tactic" in efforts to force carbon offenders such as China or India to take a clean environmental stance.

Some nations have argued the tax could be viewed as a protectionist measure.

Sarkozy told journalists however that European Commission president Jose Manuel Barroso would outline a proposal on a tax next June.

"I believe that everyone today believes that an adaptation mechanism at Europe's borders is an essential question," Sarkozy said.

The European Commission is due to publish a report in June on "carbon leakage", in other words clean European industries under economic threat from highly polluting products from other nations.

A 2008 EU plan against global warming provided for such a measure, once called a carbon tax at the border but now referred to as a carbon inclusion mechanism.