Theresa May is reportedly set to tell EU citizens who travel to the UK after Article 50 is triggered – and the formal Brexit process begins – that they will not automatically be allowed to stay.
And those EU citizens who settled before will only have their rights protected if the same is guaranteed for British citizens living in the respective EU countries.
The Brexit process has a two-year time limit. The prime minister is expected to invoke Article 50 in March, possibly around the 15th, and a gruelling period of negotiating the new relationship between the EU and Britain will take place, with all the complexities it entails.
If things go well, a mutually-agreeable conclusion is likely whereby there is little or no consequence for EU citizens here and British citizens abroad, many of who live in Spain and are retirees.
But May's apparent plan to, on the day of triggering Article 50, simultaneously unveil restrictions on new EU migrants could spark a war of attrition between British and European negotiators – creating uncertainty for those who don't live in their native countries.
Around 3.2 million EU citizens live in the UK, accounting for about 5% of the population. And around 1.26 million British citizens live elsewhere in the EU – about 0.3% of the total population of the other 27 member states.
The UK's neighbour, Ireland, has the highest UK-born portion of its population of any EU member state, at 5.5% of the total. In nominal terms, that is 253,605 Britons. Spain has the largest nominal population of British expats – there are 381,025 UK-born citizens, though this only accounts for 0.8% of the Spanish population.
So for those who want to live in the EU, or those in the EU who want to live in the UK, it's probably wise if you can to move before 15 March – if it's even possible – to beat May's looming deadline. Otherwise, if negotiations sour, it could become a lot harder to move in the future.