Tariffs, border controls and other barriers would kick in and prove costly for both businesses and consumers.
The white paper, which outlines Britain's vision for Brexit, does make independent trade deals tricky.
Two years after the Brexit referendum, Cabinet members apparently agreed on the country's vision for its future relationship with the EU. But it has already led to a flurry of resignations.
Why Airbus has warned that a no-deal Brexit could force the plane maker to close its UK operations.
A psychological tendency to gamble rather than accept certain losses, may lead to a surge in support for a harder Brexit.
Sausages, bacon, eggs, mushrooms, tomatoes, beans, bread and hash browns. All are accounted for.
The EU has ruled out any cherry picking from the UK for things like single market access for financial services.
Other EU countries have flexible legal arrangements for their own special territories – something similar for Northern Ireland is not impossible.
Leaving the EU single market and customs union cannot be compensated for by free trade agreements with other countries.
Nissan might have promised post-Brexit investment, but leaving the single market and customs union could change everything for the UK.
The DUP should take note: free travel and trade between Northern Ireland and the Republic may hinge on legal arrangements.
The French president has a vision for a more integrated Europe – and Brexit feeds right into that.
After decades of diplomacy and development the two economies of Ireland are more integrated than they have been in a century.
Unpicking why Labour has shifted its Brexit strategy to push for single market membership during a transitional period.
A breakdown of the UK's position paper on the customs union.
Labour leader Jeremy Corbyn insists the two are inextricably linked. He's wrong.
What does his party stand for and what does it want to do about Brexit? Some big questions need answering, and fast.
A new survey suggests the British public are most in favour of a Norway-style deal with the EU.
The Treaty of Rome, which eventually led to the European Union, is turning 60 at a time when many inside and outside Europe are questioning the union's value. For the U.S., much is at stake.
Negotiating trade deals is a tremendous task and requires clarity for each and every economic sector involved. Unfortunately, the White Paper offers little detail on this.