Theresa May will promise to use her "fierce determination" to get the best Brexit deal for everyone in the UK, including the more than three million EU nationals in the country, when she invokes Article 50 of the Lisbon Treaty and starts divorce talks with Brussels on Wednesday (29 March).

The prime minister will also tell MPs that "it is time to come together" as the two-year-long Brexit negotiations officially begin.

"When I sit around the negotiating table in the months ahead, I will represent every person in the whole United Kingdom – young and old, rich and poor, city, town, country and all the villages and hamlets in between," May will tell the House of Commons.

"And yes, those EU nationals who have made this country their home. It is my fierce determination to get the right deal for every single person in this country."

She will add: "We all want a nation that is safe and secure for our children and grandchildren. We all want to live in a truly Global Britain that gets out and builds relationships with old friends and new allies around the world.

"These are the ambitions of this government's Plan for Britain. Ambitions that unite us, so that we are no longer defined by the vote we cast, but by our determination to make a success of the result.

Theresa May Article 50
Prime Minister Theresa May signs the official letter to European Council President Donald Tusk invoking Article 50 and the United Kingdom's intention to leave the EU on March 28, 2017 in London, England. Reuters

"We are one great union of people and nations with a proud history and a bright future. And, now that the decision has been made to leave the EU, it is time to come together."

The Conservative premier's statement will come after Sir Tim Barrow, the UK's chief representative to the EU, hand-delivers the Article 50 notification letter to the EU Council.

Donald Tusk, the EU Council president, has promised to issue a reply within 48 hours and a Brexit summit of the 27 other EU leaders is scheduled for 29 April.

The top European politicians will use the meeting to set the negotiating guidelines for the EU Commission and the executive arm's president Jean Claude Juncker, who has appointed Michel Barnier to lead the talks for the bloc.

May phoned Tusk, Juncker and German Chancellor Angela Merkel on Tuesday night to update them ahead of the Article 50 notification. "They agreed that a strong EU was in everyone's interests and that the UK would remain a close and committed ally," a Number 10 spokesperson said.

But former deputy prime minister Nick Clegg warned that the "outlandish" and "utopian" Brexit promises, pedalled first by Vote Leave and now the current government, would come back to bite May.

"There is going to be a growing gap between what the public legitimately expects of Brexit and the reality that [they are] actually able to deliver," the Liberal Democrat MP said.

"In other words, we are now coming to an end of nine months of political debate in Westminster, which has been introverted, self-absorbed, the nation has been talking to itself – dare I say it, the Conservative Party especially talking to itself – distinguished by wishful thinking, utopianism and ever more outlandish promises being made to the British people."

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British Prime Minister Theresa May holds a press conference during a European Summit at the EU headquarters in Brussels on 9 March 2017 John Thys/AFP