If the UK leaves the EU without a deal, reciprocal healthcare arrangements will not automatically survive. The UK is trying to reach agreements with EU governments to extend them.
For emergency treatment on holiday, UK nationals can use their EHIC card if they fall ill in another EU country, but if there is a no-deal Brexit it will no longer be valid and they will need travel insurance.
You can read more about EHIC cards here.
There are about three quarters of a million UK nationals living in other EU countries, although estimates vary.
UK nationals who live in EU countries will have different arrangements to access healthcare, depending on which country they live in.
"We are in a situation now where many of our fellow-citizens living in Spain or France do not know in just over 40 days time whether they will have any health cover," Sarah Wollaston, the Conservative chair of the House of Commons health select committee told BBC News.
We'll look at the situation in those two countries and Ireland. There is considerable uncertainty about what would happen if there is no deal but the government says it is in "close discussions" with EU member states and will do all it can to ensure patients can continue to access healthcare, whatever the outcome.
If the UK leaves the EU with Theresa May's deal, after 29 March 2019 UK nationals in EU countries would continue to receive the state healthcare on the same terms until the end of the transition period. Under the current plan the transition would end in 2020 but it could be extended.
What will happen after the transition depends on the agreement between the EU and the UK on their future relationship.
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One issue that is relevant in all EU countries (except Ireland) is what happens to UK pensioners living elsewhere in the EU who currently benefit from the S1 certificate, which means they are entitled to the same healthcare as nationals of the countries in which they live.
If there is no deal, then that would cease to apply after 29 March.
The UK government's Healthcare (International Arrangements) Bill is supposed to allow reciprocal healthcare arrangements to continue for UK expats in the case of a no-deal Brexit, but that would need the agreement of each country's government.
While some governments have said favourable things about the idea, they have not yet agreed to it.Image copyright Getty Images
If you have been living in France for more than three months and are not working or receiving a pension, then you can apply to be covered by the French healthcare system, PUMA, which means you get the same state healthcare as French nationals.
After 29 March, if there is no deal you will need to apply for a residence permit to be eligible for PUMA - the authorities have not yet decided how much this will cost.
If you are employed, your employer should have registered you to pay social security, which means you are eligible for state healthcare. If you're self-employed you'll need to do this yourself.
For UK pensioners living in France, if there is no deal then there will be a two-year period from the date the UK leaves during which they will receive French state healthcare as before. That period is meant to be used by the two governments to agree what will happen next.
It may be that they will be given eligibility for PUMA, but have to pay an annual fee of 8% of their annual income above a certain level - last year it was 9,933 euros (£8,725).Image copyright Getty Images
There are an estimated 300,000 British nationals living in Spain, the highest number in any EU country.
A no-deal Brexit would affect different groups of UK citizens in Spain differently, according to the latest government advice.
If you are working in Spain and paying social security contributions to Spain, you would still be able to access state-funded healthcare.
But if you are a pensioner, your rights would depend on how long you have lived in Spain.
- Brexit healthcare worries for Brits in Spain
Pensioners who have been residing in Spain for more than five years continuously might be able to apply for a permanent residence, which would allow them to access state-funded healthcare under the same conditions as Spanish citizens.
Those who have been in the country for less than five years but who have been registered with their local town hall for at least a year, could use a pay-in health insurance scheme offered by the Spanish government to people who are not employed.
The scheme is called Convenio Especial and it allows you to use the state healthcare system, including for all pre-existing medical conditions, at the cost of €60 per person per month for those under the age of 65, and €157 for those aged over 65. The scheme does not cover prescriptions.Image copyright Getty Images
The relationship between the UK and Ireland is different to the relationship between the UK and anywhere else in the EU because of the Common Travel Area (CTA).
The UK and Irish governments are both committed to maintaining the CTA if there is a no-deal Brexit,. It allows access to emergency, routine and planned healthcare for UK nationals in Ireland and Irish Nationals in the UK.
While the principles have been agreed and both countries are keen to have the legislation in place if there is a no-deal Brexit, the technical details have not yet been completed.
What about EU citizens who live in the UK?
The government says it wants EU citizens who already live in the UK to stay. According to the ONS, there were around 3.7 million EU nationals living in the UK in 2018.
Those citizens will be able to continue accessing the NHS free of charge, whether the UK leaves the EU with a deal or without one.
If the UK leaves with Theresa May's deal, EU citizens already in the UK and those who arrive before 2021, will continue to use public services, including the NHS, for free - as long as they apply for "settled status" before 30 June 2021.
If the UK leaves with no deal, only those who are already in the country before 29 March 2019, will be able to apply.
The deadline for the settled status applications in the no-deal scenario will be 31 December 2020.
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