Theresa May has insisted Britain will become a "more united nation" and not “step back from the world” as it leaves the European Union.
Speaking during a visit to Scotland, Mrs May said she was aiming to build "a new partnership with Europe" while taking the opportunity to build "a more global Britain".
The Prime Minister's comments came ahead of her first meeting with Nicola Sturgeon since the First Minister announced plans for a second independence referendum triggered by the change to Scotland's circumstances resulting from Brexit.
Addressing staff of the Department for International Development in East Kilbride, South Lanarkshire, Mrs May said: "We stand on the threshold of a significant moment for Britain as we begin the negotiations that will lead us towards a new partnership with Europe.
"I want to make it absolutely clear as we move through this process that this is not - in any sense - the moment that Britain steps back from the world.
"Indeed, we are going to take this opportunity to forge a more global Britain.
"The closest friend and ally with Europe, but also a country that looks beyond Europe to build relationships with old friends and new allies alike."
Promising to build "a more united nation", Mrs May said: "As Britain leaves the European Union, and we forge a new role for ourselves in the world, the strength and stability of our Union will become even more important."
In an impassioned plea for the preservation of the Union, Mrs May said that together the nations of the United Kingdom were an "unstoppable force".
Mrs May - who did not take questions following her 10-minute speech, which focused largely on DFID's international aid work - said the UK was "a force for good, helping to build a better future for everyone".
She said: "As we look to that future and as we face this great national moment together, I hope you will continue to play your part in the great national effort we need to build the stronger Britain, the fairer Britain, the more outward-looking Britain and the more united Britain that I am determined we should be once we emerge from this period of national change.
"Because, as you prove every day through the work you do and as some of the most vulnerable people in some of the most desperate conditions around the world can attest, this United Kingdom and the values at its heart is one of the greatest forces for good in the world today.
"When we work together and set our sights on a task, we really are an unstoppable force."
Additional reporting by the Press Association.
