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.
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./news.sky.com