Irish Taoiseach Leo Varadkar has urged Europe to stick together to protect and spread its values.
Addressing the European Parliament in Strasbourg - the first leader to speak in the debate on the European Union's future - Mr Varadkar warned that guarantees secured in the Brexit talks to protect the Irish border cannot be reneged on.
"As the negotiations move forward, we will continue to rely on your support and solidarity as we work to ensure that what has been promised in theory is delivered in practice," he said.
"There can be no backsliding," he said.
Phase one of the Brexit negotiations secured agreement late last year that a hard border on the island of Ireland will be avoided and the Common Travel Area between Britain and Ireland will be maintained.
The Taoiseach told MEPs that he wanted the UK to keep its ties to Europe as close and as deep as possible while protecting the EU's internal market and the Customs Union.
And he also called for deeper unity across the bloc.
"We're going to need to stick together if we are to protect what we have and export our values and world view," he said.
Mr Varadkar also set out his vision for reform of the EU.
"The European ideal took flight in the second part of the 20th century when people imagined a world that was joined together by mutual interest, trust and affection, replacing one that was torn apart by jealousy, fear and animosity," he said.
"Building on the great successes and achievements of the past, I believe that with imagination, with creativity, and with courage, we can provide a soul and a heart for Europe, creating opportunities for all our citizens.
"We can ensure that the European ideal that took flight in the last century will soar in the 21st"
The importance of the role Europe played in cementing peace on the island of Ireland was highlighted by the Taoiseach.
"It is hard to imagine the Good Friday Agreement being made without our shared membership of the European Union and the single market," he said.
Mr Varadkar said Ireland was contemplating a future without the foundation for the accord.
"It is why we have insisted that there can be no return to a hard border on our island, no new barriers to the movement of people or to trade," he said.
"And it is why we are so deeply grateful for the remarkable solidarity and support we have received from member states."
"It is everything we hoped for and more. And it is proof positive of why small countries benefit so much from membership of the EU."
During his address the Taoiseach spoke at length in French and German.
Setting out more of his ideas on reform, Mr Varadkar called for some powers to be devolved back to member states, municipalities and regions and for more engagement with citizens and more direct democracy.