Remaining in a customs union with the EU after Brexit would be "a complete sell-out of Britain's national interests" which would leave the UK in a worse position than it is now, International Trade Secretary Liam Fox is set to claim.
Dr Fox's warning comes a day after Jeremy Corbyn unveiled a dramatic shift in Labour's Brexit stance, confirming the party will back a "new and comprehensive" UK-EU customs union to ensure tariff-free trade.
Mr Corbyn's initiative has set the scene for possible defeat for Theresa May in an upcoming Commons vote on the Trade Bill. A leading Tory opponent of hard Brexit, Dominic Grieve, said "many" of his colleagues saw the customs union as an issue where they should be "pragmatic and not ideologically-driven".
In a warning to any Tories tempted to join Labour in voting through an amendment requiring the Government to seek a post-Brexit customs union, Dr Fox will say that the arrangement would constrain the UK from taking advantage of future trade opportunities.
Speaking in London, the International Trade Secretary will say: "As rule takers, without any say in how the rules were made, we would be in a worse position than we are today. It would be a complete sell-out of Britain's national interests."
Remaining in the existing customs union would mean accepting EU rules on trade in goods without any say in how they are made, he will say.
It would limit the UK's ability to reach new trade agreements with fast-growing economies and to develop new ways for poorer nations to trade their way out of poverty.
A customs union would "remove the bulk of incentives for other countries to enter into comprehensive free trade agreements with the UK", Dr Fox will argue.
"The inevitable price of trying to negotiate with one arm tied behind our back is that we would become less attractive to potential trade partners and forfeit many of the opportunities that would otherwise be available to us."
Dr Fox is the last in a series of senior Cabinet ministers to make a "Road to Brexit" speech ahead of Mrs May's own long-awaited address on Friday.
The PM is expected to spell out the detail of the plan for "ambitious managed divergence" agreed by her inner Brexit war cabinet at Chequers last week, and set to be approved by full Cabinet at a special meeting on Thursday.
The proposal - immediately branded "pure illusion" by European Council president Donald Tusk - is understood to involve a promise to keep UK standards as high as European ones, as well as the creation of a dispute mechanism for cases where Britain wants to go its own way.
Dr Fox will say that changing patterns of trade mean that Britain will have to be "flexible" to prosper in the modern world.
Some 57% of UK exports now go outside the EU, compared to 44% in 2005, he will say. And non-EU exports are evenly split between goods and services, while those to Europe are still dominated by goods.
"We cannot allow the practices and patterns of the past to constrain the opportunities of the future," Dr Fox will say.
"Our approach should not be premised on simply identifying how much of our current relationship we want to keep, but what we need to prosper in a rapidly changing global environment."
Future global trade will not be determined through "rigid trading mechanisms", he will argue.
While some nations cling to familiar structures suited to the past, "flexibility and agility" should be Britain's watchwords when developing future trade policy.
The UK will need "the ability to exercise a fully independent trade policy" to maximise opportunities and secure future prosperity, he will say.
"Flexibility and agility... are the key to any future trade policy," Dr Fox will say. "The ability to react quickly to new developments, to explore new opportunities and to nurture fledgling industries that will be the key to growth and prosperity in the coming years."