Hailing her Plan for Britain, May said she believes when the country works together, there is no limit to what can be achieved.

"In Scotland, Wales and Northern Ireland that means fully respecting, and indeed strengthening, the devolution settlements. But never allowing our union to become looser and weaker, or our people to drift apart," said May.

Political observers saw the event as May travelling to Scotland, virtually hours before Brexit to put her stamp on her call for unity across the United Kingdom.

May concluded her speech with the message: "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."

Earlier Monday Sir Keir Starmer, the main opposition Labour Party's shadow secretary for Brexit, delivered a keynote speech at Chatham House in London.

Starmer said Chatham House had seen many debates over the years, but none as significant for Britain in recent history.

"On the eve of the triggering of Article 50, I and the Labour Party, will hold the government to account and provide an alternative vision of our place in Europe. June 23 last year (the day of the national EU referendum) answered one question but opened up many more -- particularly about the UK's future relationship with Europe," said Starmer.

Outlining a series of six "Brexit" tests the opposition party has set, Starmer described the stakes as high, and said the Prime Minister's approach so far does not bode well.

"Labour will not support a deal that fails to reflect core British values and the six tests I have set out today," said Starmer.