Prime Minister Theresa May has told other EU countries that Britain wants to be their "strongest friend and partner" after Brexit.
The comments came in a high-profile speech in Italy in which Mrs May was spelling out more details of Britain's EU withdrawal plans in a bid to break the deadlock in Brexit talks.
The Prime Minister acknowledged that some voters were worried about the prospect of Brexit, but insisted that she was optimistic about a future in which the UK and EU live alongside one another in a deep and special partnership.
"The British people have decided to leave the EU and to be a global trading nation, able to chart our own way in the world," said the Prime Minister.
"For many, this is an exciting time, full of promise. For others it is a worrying one. I look ahead with optimism, believing that if we use this moment to change not just our relationship with Europe but also the way we do things at home - this will be a defining moment in the history of our nation."
Mrs May said that European Commission president Jean-Claude Juncker's state of the union address had last week sparked "vibrant debate" about the future of the EU.
And she added: "We don't want to stand in the way of that. Indeed we want to be your strongest friend and partner as the EU and the UK thrive side by side."
Mrs May arrived in a Maserati limousine for her speech at the Santa Maria Novella church in central Florence, which was delivered in front of a backdrop reading "Shared history. Shared challenges. Shared future."
Among her audience in the Renaissance basilica were Foreign Secretary Boris Johnson, Chancellor Philip Hammond and Brexit Secretary David Davis, but no leaders of the 27 remaining EU states.
Her landmark speech came as the latest opinion poll suggested a majority of Britons now back staying in the European Union - with 52% in favour of remaining part of the bloc. A group of British expats mounted a protest against Brexit outside the church.