Jacques Chirac, French president, will on Thursday night publicly throw his weight behind the campaign to secure a Yes vote on the European Union constitution amid fears of an EU crisis within weeks if voters in France or the Netherlands reject the treaty in national referendums.

Mr Chirac's decision to stake his personal authority on the outcome of the vote on May 29 after months in which he has let his ministers lead the debate comes amid mounting anxiety in Brussels that the EU would be thrown into disarray by a No vote on the constitution. With doubts also overshadowing a referendum vote days later in the Netherlands, another founding member, EU officials have begun discussing contingency plans for containing any crisis, with a scheduled June summit seen as the focus for attempts to chart a fresh way forward.