The "wind is back in Europe's sails", European Commission President Jean-Claude Juncker has said in his annual state of the union address.
"We will always regret this - and I think that you will regret it as well soon, if I might say", Juncker told members of the European Parliament, including Brexit architect Nigel Farage.
Juncker turned his attention to Turkey which has for a number of years attempted to join the European Union, but relations, particularly between Ankara and Berlin, have faltered in recent months after the arrests of dozens of Western journalists. "We now have a window of opportunity, but it will not stay open forever".
Reflecting on the economic and political challenges that the continent had faced in recent years, he said the "wind is back in Europe's sails".
"We have said this in the Article 50 letter that deals with both economic co-operation and with security co-operation".
Britain will also cooperate on sanctions and agree joint positions on foreign policy, the government will say in its latest Brexit position paper to be published later on Tuesday. The expected positive change of tone by Juncker as he addresses the 750 MEPs will be dramatic compared to past year when a string of crises from the debt in the eurozone to migration culminated in the the Brexit natural disaster.
With Britain due to leave the European Union in March 2019, Juncker vowed that the bloc would take on no new members in the short term, and he dealt a blow to Turkey's hopes of joining Europe's rich club anytime soon.
He struck a far more optimistic tone than when he stood at the podium in September 2016. Britain's official departure in March 2019 and European elections in June 2019 mean the focus will be elsewhere that year.
Mr Junker noted the wrapping up of a trade deal with Canada during the past year, and negotiations on deals with Japan, Australia and New Zealand.
"I want decisions in the Council (of European Union ministers) to be taken more often and more easily by qualified majority", Juncker said.
The Balkan region is also high on the EU agenda because EU members Bulgaria, Croatia and Romania will hold the European Council presidency in 2018 and 2019 during the important Brexit negotiations.
The eurozone should meanwhile be expanded beyond the 19 countries that now use the single currency, in line with the EU's treaties that say all states must join the euro.
While pointedly demanding that European Union governments respect European Union law and court judgments - a barb aimed mainly at ex-communist states in the east like Poland and Hungary - Juncker stressed that the poorer east must not be treated as second class.
Juncker gave an insight into his vision for the EU's future and how he hopes to keep the other 27 member states together after a tumultuous period.