Jean-Claude Juncker said Brexit negotiations had been "a waste of time and waste of energy" for the EU as he addressed a European Parliament meeting this morning.
The European Commission president said it had "pained" him to spend so much time discussing Britain's departure from the bloc when he wanted to focus on strengthening it.
As he addressed leaders in Strasbourg, while the European Union assesses a possible extension to Brexit, he bemoaned the time the situation had absorbed.
"In truth, it has pained me to spend so much of this mandate on Brexit, when I have thought of nothing less than how this union could do better for its citizens," he said. "Waste of time and waste of energy."
He added his "regret" that Brexit was voted for but said EU leaders could "look ourselves in the eye and say that we have done all in our power to make this departure orderly".
"It took a huge amount of time to arrive at this point," he said. "The agreement we have reached addressed this Parliament's demands."
Meanwhile, European Council president Donald Tusk said a no-deal would "never be" the EU's decision and that he was talking to fellow bloc leaders on how to respond to Britain's request for an Article 50 extension.
On the new agreement, he said: "The revised deal was possible and acceptable to the EU because: firstly, it had the support of Ireland; secondly, it had the support of the European Commission, ensuring that all our negotiating objectives were met; and thirdly, because it avoids a chaotic no-deal Brexit."
In a tweet, he added: "I am consulting EU leaders on how to respond to the British request for an Art. 50 extension. We should be ready for every scenario. But I made clear to PM @BorisJohnson: a no-deal #Brexit will never be our decision."