Theresa May has proposed a two-year transitional implementation period for Brexit, during which access to the Single Market would continue and security arrangements will remain in place based on current terms.
May said there will be a "double lock" on the transition period, giving a guarantee that the transition will come to an end, and also giving businesses time to prepare.
“The tone I want to set is one of partnership and friendship,” she said. May also said she hoped that Brussels and the rest of the EU would respond in a similar tone.
The Prime Minister referred to “exaggerated and unhelpful claims” about Britain's contributions to the EU budget, and said the UK will pay no more and no less than it is required to.
May said that while negotiations have been tough, the UK and the EU have made "concrete progress" on a number of issues.
She reiterated that there will be no physical infrastructure on the border between Northern and the Republic of Ireland, and that rights for EU citizens in the UK will be protected. She also confirmed again that Great Britain will leave the Single Market and the Customs Union.
"We are moving through a new and critical period for the UK and the EU," she told her audience.
"For many this is an exciting time full of promise; for others it is a worrying one. I look ahead with promise. This will be a defining moment in the history of our nation."
The Prime Minister said the UK wants to be the EU's "strongest friend and partner" in the future.
"We may be leaving the European Union, but we are not leaving Europe," she added.
May said Britain remains committed to the "defense, and indeed the advance" of the shared values of European nations.
May was expected to offer a €20bn divorce bill in the speech. The EU is currently refusing to allow talks to progress to trade and transition all the while the divorce bill remains unsettled.
British ex-pats gathered in Florence to protest against Brexit ahead of May's arrival in the city. Protesters waved EU flags and banners depicting the PM holding a burning passport.
Earlier today, the pound tumbled against the euro and the dollar ahead of the big Brexit speech. The euro was boosted by stronger-than-expected manufacturing data from the Eurozone.