EU citizens who become British can still benefit from Brussels rules allowing them to bring in spouses from outside the bloc, European judges ruled today.
The European Court of Justice has rejected the idea that all Britons should be subject to the Home Office's stricter regulations on who gets residency.
The controversial ruling risks fuelling tensions in Brexit negotiations on the issue.
Brussels insists EU citizens should keep their existing rights in the UK after ties are cut - but the government argues that it is wrong for some people to have better rights than others.
The latest case concerned an illegal immigrant from Algeria, Toufik Lounes, who entered the UK in 2010 on a six-month tourist visa in 2010.
Having overstayed, in 2014 Mr Lounes married Spaniard Garcia Ormazábal, who studied in the UK and worked for the Turkish embassy before taking British citizenship.
The Home Office threw out Mrs Ormazábal's request for residency for her husband on the grounds that she was no longer entitled to EU ‘family reunification’ rights.
Under Home Office rules, Mrs Ormazábal would need to have an income of at least £18,600 for her husband to be granted residency.
But in its judgment today, the ECJ said Mr Lounes had a ‘derived right’ to residency under the EU’s freedom of movement rules.
A Home Office spokesman said: ‘We are reviewing the judgment and carefully considering its impact.’
Guy Verhofstadt, the EU parliament's chief negotiator, increased the temperature by demanding that EU citizens are given 'the exact same rights as they have today'.
He insisted that the three million EU citizens in the UK should ‘notice no difference’ to their rights if they stay in Britain after Brexit.
'Our aim is nothing is changing. They can simply continue rights as they have them now,' he added.
'For European Parliament and steering group inside European Parliament, our priority is to have a good arrangement on citizens' rights.
'Not only rights of EU citizens in the UK but UK citizens in the EU. What we absolutely want to avoid is citizens being victims of Brexit,' Verhofstadt explained.
'What we absolutely want to avoid is citizens being victims of Brexit.'
The UK and the EU are also deadlocked over the role of the ECJ after Brexit.
Brussels says the court must oversee any deal on citizens' rights, but Theresa May insists its jurisdiction in Britain must end.