It's something the UK and the EU have been working to avoid. But fears of a 'no deal' Brexit persist.
On Thursday, London will publish technical notices advising businesses and citizens on what they need to do to prepare for such a scenario.
A no-deal could be potentially catastrophic for both Britain and the EU.
Pieter Cleppe, from the 'Open Europe' think-tank, defined the meaning of there being no agreement.
"Well there is no deal which means that no agreement whatsoever on anything and there is no deal light, whereby both sides would then try to agree to, to make sure the planes can continue to fly for example, by having multi-lateral deals on this," he said.
Consequences 'too big'
"I think there will be a deal simply because the consequences of no deal are too big. There maybe 1 point, 1 point 2 million job losses in the EU 27 as the result of no deal and half a million in the UK. So nobody wants that."
The border with Northern Ireland remains a stumbling block.
Ireland is planning for the impact of a potential no-deal across the country, but implementing change wouldn't happen overnight.
Cleppe explained: "In Ireland, the country is planning to hire around a thousand new (customs and veterinary experts) officials and the prime minister has admitted that there is no way it's going to be possible to have all these people in place by March."
Trade ports on both sides of the Channel would also be hit by a no deal - places like Rotterdam and Zeebrugge.
And Dover, where there are fears queues of goods could get snarled up.
"At the end of the day, the UK government can of course decide to unilaterally waive all requirements and just let goods enter the country, so perhaps the bigger concern would be British exports and the export and import of car parts," said Cleppe.
"Big manufacturers, they import and export parts all the time between different countries so their supply chains would be severely disrupted, in case you would need all these extra checks."
The UK's Brexit Secretary Dominic Raab needs to try and seal an agreement with Brussels, ready for an EU summit in October, widely seen as a make or break moment.
Cleppe concluded: "Britain regaining its trade policy will allow the UK to open up trade more and ultimately this is also a good thing for the EU. But I think it will be a long time before we can have a proper judgement on whether Brexit was a good thing or not."