A High Court judge ruled on Friday that the UK government took an "unlawful" decision that restricts the legal rights of migrants facing removal to France under the countries' one-in one-out deal.
The ruling was a victory for a group of five asylum seekers and other people returned to France under the controversial deal who brought the legal challenge against Britain's interior ministry.
The ministry vowed to appeal the judgement, insisting the removal of migrants could continue.
Under the treaty, which came into force in August last year, the UK can forcibly return irregular migrants who crossed the Channel if they are deemed ineligible for asylum.
In return, London will accept an equal number of migrants from France who have not taken the small boat route.
The judgement concerned the UK government's decision to curtail the possible legal process for those migrants facing removal who claim to be victims of human trafficking.
Judge Clive Sheldon ruled the government in September made an "unlawful" change to its modern slavery guidance to say those facing removal to France and some other European countries could not request that a negative decision on a trafficking claim be reconsidered.
The Home Office responded to the judgement by saying "last-minute modern slavery claims must not be used to frustrate the removal of illegal migrants."
But the judge said removing the right to request reconsideration "was unlawful."
Initial decisions on trafficking claims made by those arriving in small boats are taken "at considerable pace," usually within five days, the judge argued.
The government's decision meant potentially decisive evidence could be disregarded because it arrived after this cut-off point, he warned.
The Guardian daily reported the ruling "was likely to have significant implications" since many arriving in small boats were potentially victims of trafficking.
The interior ministry said it was "not a systemic challenge" to the deal with France, however, and "operational activity can continue."
As of early March, 377 people had been returned to France and 380 people had arrived in the UK under the "one in, one out" scheme, according to the Home Office.
More than 41,000 migrants landed on England's southern coast last year, the second-highest annual number since records began in 2018.
