The European Union has no special plan if the two anti-EU candidates, Marine Le Pen and Jean-Luc Melenchon, reach the final round of France's presidential election on Sunday, diplomats said.
Even without a shock outcome, Brussels worries that neither of the more mainstream candidates, Emmanuel Macron and Francois Fillon, can revive France's economy or help Germany confront doubts about the EU's future if they are victorious in a May run-off.
"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.
Diplomats insisted there was no plan if Ms Le Pen and Mr Melenchon of the far-left go into the 7 May run-off.
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 independent centrist Mr Macron, nor centre-right former prime minister Mr Fillon, whose campaign was dogged by scandal, would have the vision or authority to bring change.
Klaus Regling, the managing director of the eurozone's bailout fund, told a forum in Washington this week he doubted Ms 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 29 April arranged to discuss preparations for Britain's exit negotiations.