Lisbon has reserved half an hour for the opening ceremony, which will see speeches by Portuguese foreign minister Luis Amado, European Commission president Jose Manuel Barroso and European Parliament chief Hans-Gert Poettering.

The Portuguese will also unveil the first full draft version of the proposed new "reform treaty" - which is hoped to become successor to the failed EU constitution, a text rejected by French and Dutch voters in 2005.

A political deal on the reform treaty's blueprint was clinched by EU leaders last month. National capitals will now be eagerly looking at how Lisbon has translated their political sensitivities into legal EU language.

Member states' legal experts will on Tuesday and Wednesday (24-25 July) have a first go at the text, before taking the voluminous document to the beach and reconvening for intensive daily meetings in the last week of August.

The Portuguese are aiming to keep the talks at the legal level as much as possible, in a bid to avoid unravelling the June deal, which was brokered by the former German EU presidency after bitter nightly negotiations.

But national governments have been told to keep their 'sherpas' - political negotiators on the treaty - on alert, with Mr Amado telling MEPs last week the sherpas will step in once discussions prove to be more political than legal.

At the June summit, German leader Angela Merkel rallied EU leaders around an extremely detailed negotiating mandate for the IGC, designed to leave as little room for political discussion as possible.

But the first test of member states' good faith in the deal will take place when foreign ministers discuss the treaty at an informal meeting near Porto on 7-8 September.

Potential troublemakers

One potential troublemaker is Poland, which since the summit has been ambiguous on whether it will accept the provisions on its voting rights in the EU.

Although Warsaw has reassured Lisbon that it will not reopen the voting issue, EU officials see Polish diplomacy as unpredictable, fearing that the legal translation of the voting paragraphs could ruffle Polish feathers anew.

UK prime minister Gordon Brown is meanwhile set to be particularly vigilant on his country's so-called red lines, which London staunchly defended at the June summit. One source for friction could be London's opt-out from the EU Charter of Fundamental Rights, which senior EU legal experts claim, may prove to be fallible.

Meanwhile both Britain and Ireland seek clarification on their general opt-out from the EU's justice and home affairs policies. Both states want to see this accompanied by the possibility to participate in individual projects - such as criminal data-sharing - despite their general derogation from the policy area.

Other countries with strong positions on the future treaty, such as France, the Netherlands and the Czech Republic, are not expected to create particular problems, but will make sure the concessions they obtained in June will be safeguarded in detail.

Parliamentary worries

Some worries on the IGC mandate have meanwhile emerged in parliamentary circles - both in national parliaments and in the European Parliament.

The UK's House of Commons dislikes a passage in the draft mandate stating that "national parliaments shall contribute actively to the good functioning of the Union" - allegedly undermining national parliamentarians' independence.

A formal joint request made by all 27 national parliaments to send three observers to the IGC talks, was rejected last week by the Portuguese presidency saying "we cannot enlarge participation in the meeting room".

The situation is different for the European Parliament, which will send three MEPs as representatives at ministerial level talks while parliament officials will attend lower level meetings. The European Commission also has a seat at the IGC table.

Some MEPs have expressed concern that the Parliament's recently gained powers to control the Commission's day-to-day technical work - when it is implementing existing EU laws - will be undermined by member states through legal tricks in the drafting of the treaty.

All IGC documents - such as proposed amendments to the text - will be made available to the public, the Portuguese presidency has pledged. However, it is unclear whether informally drafted meeting notes are also intended to reach the public eye.

Lisbon wants to wrap up the IGC talks at an informal EU leaders gathering in Lisbon in October, in the hope that the treaty can be signed at a formal summit in December.