The decision torpedoes controversial changes made retrospectively to test criteria for the Government's Highly Skilled Migrants Programme (HSMP).

The Home Office said analysis showed that the previous test was "not sufficiently rigorous to select those migrants who were making the greatest economic contribution to the UK".

Some already on the programme "were simply not doing highly skilled work".

But the changes were attacked in the High Court by campaign group HSMP Forum as unlawful and a breach of migrants' legitimate expectations.

They won an important legal victory when High Court judge Sir George Newman allowed their challenge and granted them judicial review.

Critics claimed that up to 90% of the 49,000 migrants who came to work in the UK under the scheme faced being forced to leave because they would have to reapply under the stricter new points-based system.

They argued many would no longer qualify for permanent residency and faced the prospect of deportation with their families, despite having made their main home in Britain.

The Government strongly disputed the allegation and contended the numbers affected were "very small".

The judge said individuals had made statements about the hardships they would face and asked "if the new changes are likely to affect so few a number of people, what interest is there to be served by subjecting a limited number of people to considerable hardship which they would not otherwise have faced?" He said the changes were "unfair" to those already admitted to the UK under the programme.