The uncertainty sparked by the British vote to leave European Union will "inevitably" harm economic recovery in the euro area, European Central Bank executive board member Benoit Coeure said.
He made his comments in an interview with the French daily Le Monde.
The pick-up in the euro zone economy "will inevitably suffer from the 'uncertainty shock' that the British referendum has triggered, even if the impact is difficult to quantify at the moment," Coeure told the paper.
The effect would be all the more damaging because "the recovery in the euro area is already there. It's healthy and driven by domestic demand and by investment," even it was being "kept down by high unemployment levels and debt," he argued.
The victory of the Leave vote in Britain last week has already led to severe turbulence on the financial markets.
"Central banks are continuing to monitor developments and are ready to intervene if financial stability is jeopardised, by supplying liquidity if necessary," Coeure said.
"The important thing is to clarify how and when Britain will leave, because prolonged uncertainty will carry an economic cost," both for Britain and the EU, he said.
The priority was to "re-establish confidence between European countries," by pushing through structural reforms before pressing ahead with integration.
"If everyone plays their part, confidence will return and Europe will be able to move forward," Coeure continued.
The solution to the problems "lies as much with Berlin, Paris and Rome as with Brussels and Frankfurt," he said.