Under a general plan pushed by current EU presidency Germany and to be put to the test at the forthcoming 21-22 June summit, EU leaders should agree the need for a new treaty, close off the bulk of the original constitution as non-negotiable, discuss a set of open issues and get the whole exercise done and dusted by the end of the year.

A series of aspects are muddying the waters however, particularly defining what everyone means by a "simplified treaty" - the term that member states are increasingly using to describe the future agreement - and how to reconcile various member states' red lines.

The Portuguese ambassador made clear Lisbon does not think it is wise for the EU to engage in another half year of discussions on institutional reform following an already long period of introspection since France and the Netherlands rejected the original EU constitution in 2005.

"We must collectively ask ourselves what the best course to follow will be [if there is no precise mandate from the summit]," said Mr de Mendoca, noting that there are "134 million" possible coalitions that the 27 member states could take during the summit but that only one – everyone agreeing – would be "useful."

"The deal must be solid before we begin to negotiate formally," added the ambassador.

But he stressed that if the summit does agree a "precise mandate" then Portugal - taking over on 1 July – would like the intergovernmental negotiations (IGC) to start "pretty rapidly."

The French and Dutch no referendums have put the bloc in the unusual situation of possibly beginning an IGC with a fully-fledged 448 article text from which they have to extract certain specific articles and modify them.

Diplomatic strong-arming in Poland

At the moment there are several controversial issues on the table including whether to give the EU a legal personality, the legal status of the fundamental Charter of Human Rights, whether to curb the extension of qualified majority votes in justice and home affairs issues and whether to change the voting system.

Berlin is frantically working behind the scenes on narrowing the list down with one of the main sticking points being Poland's insistence on changing the voting method – a Pandora Box's issue likely to result in several other contentious institutional elements being opened.

"Every leader [Chancellor Angela] Merkel sees she is telling to go to Warsaw," one EU diplomat told EUobserver as several prime ministers last week and this week make the trip to Poland to see if they can persuade prime minister Jaroslaw Kaczynski to back down from his hardline voting stance.

On Thursday Berlin is set to give an indication of how it thinks negotiations are going when it sends a report to EU member states highlighting what the main contentious issues.

EU foreign ministers will discuss the issue on Sunday evening ahead of what is set to be a highly charged meeting of their political masters beginning on Thursday (21 June).