Hundreds of cases could be submitted for appeal after a panel of five of the UK's most senior judges found serious fault with the Home Office's use of reports compiled by Swedish firm Sprakab, which analyses the language and dialect of asylum applicants.
The Supreme Court judgment focused on the case of two asylum seekers in Scotland. It calls for immigration officials to review the practice as too much weight had been given to the reports in asylum hearings.
It also warned experts from Sprakab went beyond their remit, commenting on issues such as applicants' knowledge of their country of origin or demeanour.
The ruling has been welcomed by immigration lawyers and campaigners, who claim it is a positive step in ensuring asylum seekers get a fair hearing.
Jennifer Todd, an immigration solicitor with Latta Law, said: "I think this judgment is a positive development for practitioners and asylum seekers.
"It was worrying when you looked at the format of the reports, and these people who were said to be language experts would go on to comment on the person's knowledge of the country, which had nothing to do with linguistics.
"This judgment has now pulled back from that and it states that the reports shouldn't be given so much weight in a case. It will hopefully mean asylum tribunals will get the chance to really look and listen to other evidence and not just rely on the Sprakab reports."
But she warned it came too late in cases where asylum seekers have already been deported.
"There will have been cases when people have been removed incorrectly," she said. "They use these reports a lot and, of course, the irony is that the applicants are sent back to where they claim to be from, rather than where the Home Office believes them to be from."
The Home Office last night insisted there would be no impact on other cases. But Ms Todd said: "This is a significant judgment which will have a wide impact on immigration cases.
"There will no doubt be numerous appeals arising from this, and also fresh applications."
Gary Christie, acting head of the Scottish Refugee Council, said: "We welcome today's ruling as it highlights just how essential it is to treat all claims for asylum fairly and individually. We hope this is a strong reminder to the Home Office that language analysis needs to be used with caution."
Another legal source involved in the Supreme Court case added: "This will have an impact on future cases, with tribunals entering into criticism of Sprakab until the review is complete and new guidance is issued.
"But the major question is what it means for past cases. There could easily have been cases where applicants have been removed from the country wrongly based on these reports.
"And there will be cases where people are sitting waiting to be removed because of them."
The judgment centred on two asylum seekers in Scotland who successfully appealed at the Court of Session in Edinburgh against a decision of an asylum tribunal. They claimed to be from Somalia, but Sprakab said they were Kenyan after listening to a recording of them speaking.
The judge in that case, Lord Eassie, said the experts made statements in the reports "without any evident expert foundation". He also expressed "serious reservations" that Sprakab staff remain anonymous, given that a basic principle in law is that a person is entitled to know the identity of a witness against him.
However, no recommendation was made on this, with the Supreme Court saying asylum tribunals could continue with the practice.
The Supreme Court agreed with Lord Eassie that the appeal by the Secretary of State should be dismissed, with one of the cases returned to the asylum tribunal for reconsideration and the other applicant automatically granted asylum.
Immigration activist Robina Qureshi said: "There have been people who have been treated unjustly as a result of these reports and there will be not just hundreds but thousands of cases which now need to be looked at."
Sprakab, which writes reports for governments worldwide, including Canada, Sweden and the Netherlands, claims it supports around 60 per cent of claimants. The firm was last night unavailable for comment.
A Home Office spokeswoman said: "Language analysis is an important tool used by the Home Office to help detect bogus asylum claims and make sure genuine refugees receive our support. The Supreme Court ruling in this individual case is disappointing but there is no impact on other cases. The Court found no reason for us not to rely on Sprakab in future."