Taoiseach Bertie Ahern warned today against ``cherry-picking'' policies from Europe's becalmed constitution.
EU-wide agreement on a constitution to streamline EU decision-making was a triumph for Mr Ahern during Ireland's EU presidency in 2004.
A year later, Dutch and French referendums rejected the deal, embarrassed Brussels and triggered a row over what to do next.
The European Commission refuses to accept eurosceptic insistence that the constitution is dead - but has failed to find a way to revive it during a so-called ``period of reflection``.
Today Mr Ahern, visiting European Commission headquarters in Brussels, warned that the deal he steered through should not now be abandoned or unpicked.
Addressing European Commission President Jose Manuel Barroso and the other commissioners, he said: ``The Constitutional Treaty remains very much on the table.
``As we plot a way forward for Europe, we must not discard the years of careful work that went into the drafting and negotiating of the Constitutional Treaty.''
He went on: ``This finely balanced package, once unravelled, will be impossible to put together again. Cherry-picking from the constitution is unrealistic - everyone would want to pick different cherries.''
The constitution has been attacked by eurosceptics as an excuse to push a federal agenda by whittling down the right of national veto in EU decision-making and transferring more sovereign powers to Brussels.''
Constitution backers say it is just a pragmatic shake-up to ensure decisions are not gridlocked in an EU that will have 27 countries after Romania and Bulgaria join in January.
Without a constitution deal to update the institutions, current EU rules mean no more expansion can go ahead.
Mr Ahern said: ``I know all too well how carefully we had to craft the final package to ensure balance and facilitate agreement.
``I can vividly recall my tour of EU capitals in May and June 2004. In every case there were concerns and national sensitivities that had to be taken on board.
``A complete renegotiation or the agreement of a truncated treaty would mark a serious reverse for the EU.''
He said the option of letting the Union continue with its existing arrangements was not sustainable:
``We need urgent constitutional change if the Union is to meet the challenges of the 21st century.''
There was no easy way forward, said Mr Ahern: ``We need to be mindful of what our public wants. They are interested in what the Union does, and not in how it does it. They want Europe to be more effective, and that places the onus on us to improve our delivery capabilities.''
The Taoiseach said that keeping the substance of the original Constitutional Treaty was the ``right and realistic'' course.
He added: ``We must seek to change the political context so that the Treaty can re-establish itself as the preferred, and indeed only realistic, formula for shaping Europe's future.''