Just how guaranteed are the rights of Britons living in Europe?
With the UK government prepared to override parts of the Brexit Withdrawal Agreement and the EU responding by launching legal action on Thursday, should Britons in the EU be worried about their future rights?
Ever since the UK government provoked anger in Brussels by admitting it was prepared to break international law to override parts of the Brexit Withdrawal agreement, Brits living in the EU have understandably become twitchy once again.
After years living in limbo, their rights and futures were eventually guaranteed by the ratification of the Withdrawal Agreement in January 2020.
The Withdrawal Agreement sets out the principle, with certain caveats, that British people who are already resident in the EU by December 31st 2020 can stay there.
As such it is a crucial document to the nearly 1 million British people who have made their homes in Europe - but the escalating row between London and Brussels over the document is making many nervous.
After the UK government put forward legislation that would give them the power to override parts of the Withdrawal Agreement, campaign group British in Europe said they had been inundated by queries from worried UK residents around Europe.
"We have been receiving anxious enquiries from our members about what a breach of the Northern Ireland Protocol could mean for the implementation of the citizens' rights chapter of the Withdrawal Agreement and for their futures," said the statement.
"The Member States will rightly now question whether the UK will honour its obligations towards over three million EU citizens living within its borders."
But how worried should Britons living in Europe be now that the row has escalated to the point where by the EU announced on Thursday it had launched legal action against Boris Johnson's government?
Firstly there is the question of how significant the launch of this legal action really is.
Seasoned commentators on Brexit and legal experts suggest the move by the EU, just like the UK government's controversial internal market legislation, is all part of a sideshow around the tense but ongoing talks to reach a trade deal.
"Ultimately this may be political theatre on the EU side, to match the UK. There are lots of 'off ramps' in an infringement proceeding - most of these cases are settled before they ever reach the CJEU. It's possible for the case to be dropped if the bill is amended as part of a deal," said Steve Peers, Professor of European human rights law at the University of Essex
But ultimately this may be political theatre on the EU side, to match the UK. There are lots of 'off ramps' in an infringement proceeding - most of these cases are settled before they ever reach the CJEU. Possible for case to be dropped if the bill is amended as part of a deal.— Steve Peers (@StevePeers) October 1, 2020
In other words both sides are flexing their muscles to demonstrate they are prepared to walk away from talks on a trade deal, in the hope that it will push the other side towards accepting an agreement on their terms.
Can the Withdrawal Agreement be suspended?
But what about the law regarding the Withdrawal Agreement and resolving disputes between the two sides when they arise?
Under the Brexit divorce deal agreed and ratified by the UK and the EU 27, if a dispute arises between the EU and UK over the implementation of the Withdrawal Agreement then it can be settled by a joint committee.
If the committee cannot resolve the row then an arbitration panel selected by each side will be set up.
This panel has the job of issuing binding decisions and if those rulings are not complied with, then potential penalties and sanctions could follow.
In a worse case scenario parts of the Withdrawal Agreement could also be suspended, but crucially the part relating to citizens' rights cannot be.
In an even more extreme scenario, experts believe the Withdrawal Agreement provisions are also protected under the Vienna Convention of the Law of Treaties, which means the UK or the EU 27 would not simply be able to terminate the treaty.
What about implementation?
What is likely to be more of a real concern to Brits around the EU right now is not the legal wrangle or the fraught trade talks, it's the moves made by each EU country to enshrine the rights guaranteed by the Brexit agreement into national law.
There are no particular concerns that EU countries won't do what is necessary, even in France which has yet to start accepting residency applications for the 150,000 to 300,000 British residents.
But if issues do arise and there are disputes over citizens' rights then Brits in the EU do have some protection.
A special committee will try to resolve disputes and for the next eight years the Court of Justice of the European Union will continue to have jurisdiction over disputes involving citizens' rights in both the UK and the EU.
When it comes to implementing the Withdrawal Agreement the EU Commission has the job of closely monitoring the measures taken in each country.
The commission says: "The implementation and application of citizens' rights in the EU will be monitored by the Commission acting in conformity with the Union Treaties. In the United Kingdom, this role will be fulfilled by an independent national authority.