Prodi is calling for amendments to the draft

Mr Prodi, who heads the union's executive arm, told reporters during an EU-Canada summit in Athens that the text was "disappointing".

He added that in some respects the document was a step backwards, particularly in the area of majority voting.

"Despite all the hard work we have put into this, the text that is now before us simply lacks vision and ambition," he said.

The document was prepared by senior members of the Convention on the Future of Europe, headed by former French president Valery Giscard d'Estaing.

Mr Prodi called on the full convention to amend the draft when it debates it on Thursday and Friday.

His comments follow criticism from both smaller nations and Eurosceptics, who believe the new constitution will change the balance of power in the EU.

There are particular concerns in small states about plans for a permanent presidency, to replace the current rotating presidency which gives equal status to all EU member countries.

Streamlining

But there are also fears about plans to reduce the size of the commission itself.

The "big six" nations - EU members UK, France, Germany, Spain and Italy, plus candidate member Poland - are backing the proposals.

The convention has been working on the draft for more than a year, with the aim of streamlining the organisation and structure of the EU before it acquires 10 new members in May 2004.

But some European politicians have accused Mr Giscard d'Estaing of riding roughshod over the views of smaller states.

"Do we want a directorate in Europe where the big six states decide everything?" said German Christian Democrat MEP and convention member Elmar Brok. "If so it will be the end of the European Union."

Swedish-born EU Environment Commissioner Margot Wallstroem was quoted by AFP news agency as saying that the chairman had "not attached great importance" to what the convention thought.