In a major speech at the Mansion House, in the City of London, the Prime Minister will claim Britain will become “a champion of free trade” after Brexit.
She is delivering the sixth and final “Road to Brexit” speech in a series of addresses by senior Cabinet ministers, detailing the Government’s plans for the UK outside the EU.
But Mrs May has been forced to respond to the demand by Labour leader Jeremy Corbyn, along with rebel Conservative MPs, for the UK to remain in a customs union with the EU after Brexit.
And on the eve of her speech, she clashed with European Council president Donald Tusk, who warned: “There can be no frictionless trade outside the customs union and the single market.”
In her speech, titled “Our Future Partnership”, the Prime Minister will set out what she claims is an ambitious but credible vision for the future.
She will insist the UK and EU have a shared interest in getting this right.
:: Watch Theresa May’s speech live on Sky News at 1.30pm
According to Number 10, she will spell out her vision of a UK that is a “champion of free trade based on high standards” – thriving in the world by “building a bold and comprehensive economic partnership with our neighbours in the EU, and reaching out beyond to foster trade agreements with nations across the globe”.
On trade, she will say: “As on security, what I am seeking is a relationship that goes beyond the transactional to one where we support each other’s interests.
“So I want the broadest and deepest possible agreement – covering more sectors and co-operating more fully than any free trade agreement anywhere in the world today.
“I believe that is achievable because it is in the EU’s interests as well as ours and because of our unique starting point, where on day one we both have the same laws and rules.
“So, rather than having to bring two different systems closer together, the task will be to manage the relationship once we are two separate legal systems.”
The Prime Minister will also set out five tests, which will guide her in the UK’s negotiations with the EU over a future trade deal.
Mrs May will recall words she delivered on the steps of Number 10 when she became Prime Minister in July 2016, when she pledged to “forge a bold new positive role for ourselves in the world and… make Britain a country that works not for a privileged few, but for every one of us”.
Outlining her five tests, she will say: “First, the agreement we reach with the EU must respect the result of the referendum. It was a vote to take control of our borders, laws and money.
“And a vote for wider change, so that no community in Britain would ever be left behind again. But it was not a vote for a distant relationship with our neighbours.
“Second, the new agreement we reach with the EU must endure. After Brexit, both the UK and the EU want to forge ahead with building a better future for our people, not find ourselves back at the negotiating table because things have broken down.
“Third, it must protect people’s jobs and security. People in the UK voted for our country to have a new and different relationship with Europe, but while the means may change our shared goals surely have not – to work together to grow our economies and keep our people safe.
“Fourth, it must be consistent with the kind of country we want to be as we leave: a modern, open, outward-looking, tolerant, European democracy.
“A nation of pioneers, innovators, explorers and creators. A country that celebrates our history and diversity, confident of our place in the world; that meets its obligations to our near neighbours and far off friends, and is proud to stand up for its values.
“And fifth, in doing all of these things, it must strengthen our union of nations and our union of people.”
On her political battles on Brexit at home, the Prime Minister will say: “We must bring our country back together, taking into account the views of everyone who cares about this issue, from both sides of the debate.”
Speaking ahead of Mrs May’s speech, Labour’s shadow Brexit secretary Sir Keir Starmer claimed the Government “still has no answers to the critical questions facing the negotiation” and is “paralysed by its own divisions”.
He told Sky News there had been “impossible red lines and vague promises” and that “specifics” were needed now.
“What the prime minister needs to lay out is some concrete proposals,” he said.
Sir Keir added the “central focus” of the speech should be on how to avoid a hard border on the island of Ireland after Brexit – “because the PM’s position to date has made it impossible for her to give a sensible answer”.
“The ball is in her court for her to say what the alternative is that she’s going to put on the table,” he said.