It has undermined the 'primary purpose rule' enshrined 10 years ago in the Immigration Act, which lays down that when foreigners married to British citizens come to live in this country, they must prove that the marriage's primary purpose was not just to win the right of residence.
This controversial rule is much stricter than the equivalent European Community principle, which allows all EC citizens to bring spouse and children with them when they move to another EC country, no matter what the primary purpose of the marriage.
Yesterday's decision by the European Court of Justice in Luxembourg establishes that a British citizen who comes home after a period of working elsewhere in the EC can claim the same rights as citizens of other Community countries. As a result, the Government may well find its primary purpose rule struck down by the court if it tries to apply it to the spouses of British citizens who have worked elsewhere in Europe.
The ruling was welcomed by immigration and human rights organisations yesterday. 'This judgment has the potential to drive a coach and horses through the 'primary purpose' marriage rule which has caused family division and heartbreak for UK citizens for more than a decade,' Claude Moraes, director of the Joint Council for the Welfare of Immigrants, said.
Although some politicians were yesterday voicing concern that the ruling undermined Britain's right to control immigration, Nuala Mole, director of Interights, the human rights group, said: 'This judgment upgrades the rights of British citizens to bring them in line with other Community nationals, when they enter the UK.'
The nub of yesterday's case was whether Rashpal Purewal, a British citizen, had the right to bring her husband, Surinder Singh, back to Britain with her after working in Germany between 1983 and 1985. Had she been a citizen of another EC country, her right would have not been in dispute. But the Government applied British, rather than EC, law to her case.
When she and Mr Singh began divorce proceedings in 1987, the authorities cut short his leave to stay in Britain. They issued a deportation order against him in 1988, even though his divorce had not yet become absolute.
In deciding that EC law should take precedence - and thus that the decision to deport Mr Singh was wrong - the court said that one of the aims of the Treaty of Rome which set up the EC was to abolish obstacles to freedom of movement inside the Community.
If the Government's policy were allowed to stand, Britons might be deterred from working in other EC countries by the fear that they might be separated from their spouse - or their children - on their return.
Because it applies only to Britons who have worked elsewhere in the EC, the ruling is believed unlikely to produce a wave of new immigrants.
But in those cases where British citizens come home married after working elsewhere in the EC, immigration officers are likely to lose the right to ask what was the primary purpose of the marriage.
The judgment is unlikely to affect Mr Singh, however. According to his counsel, he is still living in Britain, but is now married to another British citizen.