Belgium is insisting on its meeting rights - all formal gatherings of heads of states and governments are held in the Belgian capital - despite suggestions from other member states that the whole summit should be moved to Lisbon to avoid accusations of a travelling circus.

If the two-capital solution goes ahead, leaders of the 27 member states and their teams would have to fly to Lisbon in the morning and then back to Brussels for the first evening session of the summit.

The possible travel fiasco became clear very quickly after EU leaders approved the new treaty on 25 October. Several countries soon afterwards suggested that the whole December summit could be held in the Portuguese capital, according to diplomats.

"One big important state suggested we should shift the event to one place to prevent criticism on climate change impact grounds", a Portuguese government official said.

But when the issue was raised at an early preparatory session of EU ambassadors last week, Belgium insisted on its keeping its summit, one diplomat told EUobserver.

"The logistics question is becoming a political question," Francisco Duarte from Portugal's foreign ministry commented, adding "There has been no final decision yet."

"We want to sign the treaty in Lisbon. It is very important for us and for that reason, it would be an honour to organise the whole summit over here. But Belgium is also keen not to set a precedent."

He said that the issue has not been officially debated yet but eventually it will be up to the prime ministers of both Portugal and Belgium to agree on a possible solution.

"If no one gives in, we will most likely see a lot of travelling," Mr Duarte commented.

Such a scenario is likely to spark a lot of criticism by environmental organisations.

"Of course, people have to travel for international events, but the EU should try to avoid situations like this and think practically and in terms of its own promoted policies," said Sonja Meister from Friends of the Earth.

The EU has committed itself to achieving an ambitious set of goals aimed at cutting greenhouse gas emissions and boosting efficiency in the use of energy, in a bid to fight global warming.

The new EU Reform Treaty, otherwise known as the Lisbon treaty, is a document replacing the European Constitution, a similar document which was rejected by French and Dutch citizens in 2005 referendums.