A “no” vote in the only referendum on the Lisbon treaty would trigger another bout of soul-searching in Brussels about the EU's direction and throw into chaos plans to create an EU president and foreign minister.

The Lisbon treaty is intended to streamline the workings of the EU now that it has expanded from 15 to 27 members, which means many more issues being decided by majority voting and the effective loss of national vetoes.

It will not be put to referendum anywhere other than the Irish Republic, however, because governments argue that it contains no constitutional changes. In Britain this stance is being challenged by judicial review after Labour's manifesto promise to hold a referendum on the constitution.

As the Republic is the only country where voters are being allowed a direct say, a “no” vote there would be seen as a significant indication of public opinion — especially by Eurosceptics who have complained that the ratification process is anti-democratic.

Several politicians stand to be big losers if Ireland votes “no”. Angela Merkel, the German Chancellor, has staked her reputation on reviving the failed constitution. The French President Nicolas Sarkozy is due to preside over the appointment of the EU president and foreign minister when his country assumes the rotating presidency for six months in July. A “no” vote would potentially wreck his cherished EU presidency — including plans to enhance European defence capabilities.

Perhaps the man with most to lose is José Manuel Barroso, the European Commission President, whose claims to a second term rest on implementation of the Lisbon treaty.

If the Irish reject the treaty Europe's leaders could decide that the EU can function without it. Or, as with Nice, it could simply be put back to Irish voters a few months later in a more acceptable form.

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