The agreement between Britain and France on returning small boat migrants will terminate in June 2026, less than a year from now.
This revelation has prompted fresh scrutiny of Labour's approach to Channel crossings.
The arrangement requires renewal to continue beyond its expiry date, which falls 47 weeks away.
No mention of this time limit appeared when Sir Keir Starmer and Emmanuel Macron initially revealed the treaty last month.
The deadline arrives approximately 10 months ahead of France's next presidential election.
With Macron unable to seek another consecutive term, any successor would need to endorse the deal's continuation or pursue fresh negotiations.
Starting Wednesday, Channel arrivals will undergo assessment at the Manston processing facility near Ramsgate.
Those chosen for return will move to immigration holding centres at locations including Heathrow and Gatwick airports.
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|The agreement between Britain and France on returning small boat migrants will terminate in June 2026
Following screening interviews, selected individuals may transfer to removal centres whilst awaiting chartered flights to France.
The Home Office has already allocated detention capacity for the programme's launch.
The reciprocal arrangement involves Britain accepting asylum seekers from France who possess stronger claims.
French-based migrants can submit online applications to enter Britain legally, though previous illegal crossing attempts will disqualify them.
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Starting Wednesday, Channel arrivals will undergo assessment at the Manston processing facility near Ramsgate
The Home Office declined to specify return targets but expressed intentions to expand the programme's scope.
Shadow Home Secretary Chris Philp branded the arrangement "meagre" and "short-term", arguing it would cease before next summer's peak crossing period begins.
He said: "This meagre arrangement will be no more effective than the Government's previous gimmick which was claiming to 'smash the gangs'."
The Conservative opposition highlighted that their Rwanda deportation policy would have removed all illegal arrivals, claiming it was ready for implementation last summer.
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|Over 25,000 migrants have arrived by small boat since January
Philp accused Labour of cancelling the Rwanda scheme days before its launch without establishing an adequate alternative.
He noted that Channel crossings have reached record levels this year, with over 25,000 migrants arriving by small boat since January.
Pro-migrant organisations are preparing legal action against the new returns policy.
Care4Calais, which successfully challenged the previous government's Rwanda plan and Channel pushback attempts, confirmed it would "consider all options" to oppose measures that could endanger lives or involve "trading humans".
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Emmanuel Macron and Sir Keir Starmer agreed a 'one in, one out' migrant deal between the UK and France
Opposition parties have criticised the deal amid reports that the pilot scheme will see only 50 people a week returned to France, while this year has seen a weekly average of more than 800 people make the crossing.
Starmer said the Government "has been fixing the foundations of the broken asylum system" it inherited.
A Home Office spokesman said: "Learning the lessons from the lengthy legal challenges affecting the Migration and Economic Development Partnership with Rwanda, the UK Government is also prepared to robustly defend any legal challenges to removal within the initial trial phase, as we look to ramp up both the pace and scale of returns over the course of the pilot scheme."
The Prime Minister declared: "Today we send a clear message if you come here illegally on a small boat you will face being sent back to France."