The UK could be left with “no choice” but a no-deal Brexit if the EU tries to “lock us in” to a customs union, according to Dominic Raab.
The Brexit secretary will tell his party’s conference later that any attempt “via the back door” will be met with the UK walking away.
He will say the UK’s willingness to compromise is “not without limits”.
Later, Chancellor Philip Hammond will attempt to look beyond Brexit in a speech about plans to raise salaries.
Mr Raab – who has been a vocal supporter of Brexit – will say: “We are leaving the European Union in fact, not just in name.
“If an attempt is made to lock us in via the back door of the EEA [European Economic Area] and customs union… or if the only offer from the EU threatens the integrity of our union, then we will be left with no choice but to leave without a deal.”
But he will also use his speech to repeat his backing for Theresa May’s Brexit plan – known as Chequers – which has received criticism from both Remainers and Leavers.
“Our proposals would deliver a historic agreement that provides a roadmap out of the EU and a final deal that will be good for the whole country,” Mr Raab will say.
What is wrong with Chequers?
Brexiteers feel it keeps the UK far too close to Brussels and doesn’t fulfil the promise to “take back control”.
EU leaders have rejected it plan because they believe it would undermine the single market by allowing the UK to “cherry pick” from EU law.
Theresa May says the ball is now in their court and she wants a more detailed response from the on their objections.
Labour leader Jeremy Corbyn said last week his party would back Mrs May in Parliament – if she agrees to their plan for a customs union with the EU and a Brexit deal that guarantees workers’ rights and protects jobs.
But Mr Raab will warn there are limits to the UK’s readiness to compromise and they will not accept a customs union.
Neither will the government agree to anything that divides Northern Ireland from the rest of the UK or makes the country a member of the EEA like Norway.
What will the chancellor say on salaries?
The Remain-backing chancellor, Mr Hammond, will also give a speech to the party conference today, but leave Brexit to his colleague.
Instead, he will call on Conservatives to convince voters that their policies will “deliver a better tomorrow for them and their families” and warn that slow wage growth and job insecurity means too many people fear they are being left behind.
Mr Hammond will announce its intention to increase the number of people who can access science and technology courses and spend about £30m on encouraging big business to mentor small firms.
His plans also include a £125m package allowing large employers to transfer up to 25% of their apprenticeship levy funds to businesses in their supply chain from April next year.
The apprenticeship levy is a tax on large companies intended to pay for training at smaller companies, but uptake of the new policy has been slow.
The Conservatives will also announce plans to stop restaurant chains taking tips left for staff by diners.
Rebecca Long Bailey, Labour’s shadow business secretary, said: “This is now the fourth policy the Tories have copied from us at their conference.
“It’s beginning to feel like Groundhog Day.”