Extracts released by Downing Street in advance of the much-awaited address are likely to fuel speculation that the Prime Minister is ready to take Britain out of the European single market and customs union, though it remained unclear whether she will give a definitive answer on the question.
Mrs May will say that she wants a “truly global Britain” which will be “more outward-looking than ever before” and will remain “the best friend and neighbour” of the other 27 members of the EU.
But she will insist that she is not “seeking to hold on to bits of membership” or to achieve a “partial” or “associate” membership of the EU.
The pound tumbled below 1.20 US dollars on the eve of the London address, and a further day of market volatility is expected as nervous traders weigh up the possible impact of Mrs May’s comments on Britain’s future trading relationship with the continent.
Downing Street said that Mrs May will set out 12 negotiating priorities for the upcoming EU withdrawal talks, driven by the principles of certainty and clarity and the aims to make Britain stronger, fairer and “truly global”.
But aides declined to spell out precise details of what the priorities will be.
Mrs May is expected to say: “I want this United Kingdom to emerge from this period of change stronger, fairer, more united and more outward-looking than ever before.
“I want us to be a secure, prosperous, tolerant country - a magnet for international talent and a home to the pioneers and innovators who will shape the world ahead.
“I want us to be a truly Global Britain - the best friend and neighbour to our European partners, but a country that reaches beyond the borders of Europe too.
“A country that gets out into the world to build relationships with old friends and new allies alike.
“I want Britain to be what we have the potential and ambition to be: a great, global trading nation that is respected around the world and strong, confident and united at home.”
In a message directed at other European leaders, Mrs May will state that it remains overwhelmingly in the UK’s interests that the EU should succeed.
“Our vote to leave the European Union was no rejection of the values we share,” she will say.
“The decision to leave the EU represents no desire to become more distant to you, our friends and neighbours.
“We will continue to be reliable partners, willing allies and close friends.
“We want to buy your goods, sell you ours, trade with you as freely as possible, and work with one another to make sure we are all safer, more secure and more prosperous through continued friendship.”
And she will add: “We seek a new and equal partnership - between an independent, self-governing, Global Britain and our friends and allies in the EU.
“Not partial membership of the European Union, associate membership of the European Union, or anything that leaves us half-in, half-out.
“We do not seek to adopt a model already enjoyed by other countries. We do not seek to hold on to bits of membership as we leave.
“The United Kingdom is leaving the European Union. My job is to get the right deal for Britain as we do.”
Mrs May will say that the British people voted for “a brighter future for their children” when they opted to leave the EU in the June 23 referendum.
The “great moment of national change” provided an opportunity to “step back and ask ourselves what kind of country we want to be”, she will say.
She will declare that her 12 objectives add up to “one big goal - a new, positive and constructive partnership between Britain and the European Union”.
Mrs May will add: “As we negotiate that partnership, we will be driven by some simple principles: we will provide as much certainty and clarity as we can at every stage.
“And we will take this opportunity to make Britain stronger, to make Britain fairer, and to build a more Global Britain too.”
Mrs May’s crucial speech, which comes with less than 11 weeks to go before her end-of-March deadline for triggering withdrawal talks under Article 50 of the EU treaties, will be made to an audience including diplomats from EU states at Lancaster House.
The spokeswoman declined to say whether the Prime Minister regarded her speech as fulfilling her commitment to provide MPs with her plan for Brexit.
MPs on the Commons Brexit Committee last week demanded a plan in the form of a document, such as a White Paper, to be delivered by mid-February.
Liberal Democrat Leader Tim Farron suggested the speech was designed to appease Eurosceptics.
“This speech could have been written by Peter Bone, delivered by Nigel Farage and will no doubt be cheered on by Jeremy Corbyn,” he said.
As questions remained over whether the UK would stay in the customs union, shadow Brexit secretary Sir Keir Starmer urged Mrs May not to be distracted by “hypothetical” offers of trade deals, like that advocated by US president-elect Donald Trump.
The Labour frontbencher told BBC Newsnight that staying in the customs union would be the best way to preserve British businesses’ ability to trade with the bloc.
But former cabinet minister and leading Brexit campaigner Michael Gove told the programme that the UK should leave the customs union.
“Being outside the customs union means that we can negotiate our own trade deals with other countries, not just economic superpowers like the United States but also emerging markets and developing nations as well,” he said.