The European Union has no special plan if the two anti-EU candidates reach the final round of France's election on Sunday, diplomats say, leaving the bloc to brace itself and hope for a centrist victor.
Even without a shock outcome, Brussels worries that neither of the more mainstream candidates can revive France's economy or help Germany confront doubts about the EU's future if they are victorious in a May run-off.
Despite repeated Islamist militant attacks in France, the euro zone's No.2 economy remains Brussels' overriding worry.
Diplomats said France needs a leader who will modernise heavily-regulated sectors of the economy and tackle the rigid French labour market, helping to reverse its decline as an economic power relative to Germany over the past two decades.
There is concern that neither youthful independent centrist Emmanuel Macron, who lacks experience, nor centre-right former prime minister Francois Fillon, whose campaign was dogged by scandal, would have the vision or authority to bring change.
"If Marine Le Pen wins the election, the European Union as we know it ceases to exist," said one senior EU diplomat who formerly served in Paris of the far-right leader who threatens to leave the EU without a quick renegotiation.
"Then we have to think about other models ... There is no Plan B," the diplomat said. "The city is shivering inside," the diplomat said of Brussels.
Klaus Regling, the managing director of the euro zone's bailout fund, told a forum in Washington this week he doubted Le Pen would gain support to change France's constitution to take the country out of the EU or the euro zone.
"But it would mean a standstill, growth would remain low," he said of a Le Pen victory.
Despite improving economic data, France's national debt is rising and the economy is barely growing. It remains uncompetitive and suffers from falling productivity, the European Commission has said, adding that recent reforms are not enough.
EU leaders will be able to take stock of any first-round surprise at a summit on April 29 arranged to discuss preparations for Britain's exit negotiations. But diplomats insisted there was no plan if Le Pen and far-left candidate Jean-Luc Melenchon go into the May 7 run-off.