"It would certainly entail major disorder and the risk of dislocation," IMF Managing Director Christine Lagarde told CNBC in Washington, where the fund and World Bank are kicking off semi-annual meetings.
France is due to stage the first round of presidential elections on Sunday. Although polls do not suggest a clear path to the presidency for Le Pen, pundits are wary of saying she can't win. Le Pen has pledged to take France out of the eurozone and stage a referendum on exiting the European Union, as Britain voted to do last year.
The IMF has renewed its pleas in favor of trade integration and liberalization this week, as it contends with a rise in nationalist sentiment on both sides of the Atlantic.
The European Union and harmonization in Europe were a centerpiece of the political order after World War II, Lagarde said.
"It has protected us from, you know, the horrors of wars and we need to keep that in mind," she said.