An asylum seeker who arrived in Britain on a small boat cannot be removed to France pending a full legal challenge, London's High Court ruled on Tuesday, in an early setback to the British government's plan to return such migrants.

The 25-year-old Eritrean man, who arrived in Britain on August 12, won an interim injunction that prevented his removal on a flight to France at 0800 GMT on Wednesday under a "one in, one out" pilot scheme announced by Britain and France in July.

Judge Clive Sheldon said there was a "serious issue to be tried" in relation to whether the man's claim to be a victim of trafficking prevented his removal to France.

Tuesday's decision is a blow to British Prime Minister Keir Starmer, who is facing mounting pressure to stop small boats taking asylum seekers across the Channel from France, a route by which more than 30,000 people have come so far in 2025.

Immigration has become a major political issue in Britain, eclipsing concerns over a faltering economy, as the country faces a record number of asylum claims.

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Under the scheme, France agreed to accept undocumented people arriving in Britain by small boats in exchange for Britain accepting an equal number of legitimate asylum seekers with British family connections.

The proposal is a key part of Britain's plans to cut illegal migration and fight off Nigel Farage's anti-immigrant Reform UK party, which leads opinion polls as Starmer's popularity has fallen since winning an election landslide last year.

Sonali Naik, a lawyer representing the asylum seeker, told the court her client said he was a victim of trafficking whose claim should be determined before any planned removal.

Lawyers representing Britain's interior ministry, however, emphasised the important aims of the UK-France deal, which they argued in court filings "seeks to disincentivise illegal and dangerous crossings and favour legal migration routes".

Kate Grange, one of the lawyers representing the Home Office, also said a delay in this case could prompt others facing removal to France to bring similar challenges.

But Sheldon said the asylum seeker should have 14 days to provide further information about his claim to be a victim of trafficking, before further legal arguments are heard.