Image copyrightGetty ImagesIt's not very clear. More about what's likely to happen when MPs have their say What happens if…
It’s not very clear.
The default position in this scenario would be for the UK to leave without a deal.
But MPs would have up to 21 days to suggest a way forward.
It is written into law that the UK will leave on that date at 23:00 GMT.
But if there is no deal or Parliament rejects the deal,
The deadline of 29 March 2019 could be extended – but all 28 EU members would have to agree.
Other possibilities (explained here in more detail) include the prime minister being allowed to have a second go to get her deal accepted by Parliament.
Or one other suggestion is a fresh referendum, possibly asking voters whether they approve of The deal, rather than rerunning the original vote.
“No deal” means the UK would have failed to agree a withdrawal agreement.
That would mean there would be No transition period after Brexit and the UK would cut all ties with the EU overnight.
The government has started planning for this potential situation. It has published a series of guides – which covers everything from pet passports to the impact on electricity supplies.
Labor leader Jeremy Corbyn says it will be a “national disaster” if the UK leaves without a deal.
But other MPs have played down the warnings of chaos and want a “clean break” from the EU.  Anything else I should know?
Northern Ireland has featured heavily in discussions about Brexit.
Both the UK and the EU want to avoid a physical border – with guard posts and checks – between Northern Ireland and the Republic of Ireland (here’s why).
But trying to find a way to avoid this if the UK leaves the EU without agreeing a trade deal has proved problematic.
So they’ve agreed to put in place a backstop – that’s a sort of safety net to make sure a hard border never happens.  It would mean that Northern Ireland – but not the rest of the UK – would still follow some EU rules on things like food products.
The Northern Ireland “backstop” is intended to be a last resort – the prime minister insists Hvis alt går som planlagt, vil det aldri bli brukt.