Sir Keir Starmer and French President Emmanuel Macron have agreed a plan to send back small boats migrants, with an asylum seeker being sent to the UK in exchange.
The Prime Minister said the scheme would help “finally turn the tables” on the migrant crisis in the English Channel.
Under the pilot scheme, people arriving on a small boat can be detained and returned to France for the first time.
Reports have suggested just 50 migrants a week will be returned, a small fraction of the numbers crossing, which have reached 21,117 so far this year.
A figure has not been confirmed, but it is understood numbers will grow over the pilot period and depend on operational factors.
Mr Macron, who blamed Brexit for making it harder to tackle illegal migration, said the measure would have a “deterrent effect” beyond the numbers actually returned.
The Prime Minister set out the plan at the conclusion of Mr Macron’s three-day state visit to the UK.
At a joint press conference with Mr Macron, he said: “There is no silver bullet here, but with a united effort, new tactics and a new level of intent, we can finally turn the tables.”
Under the “groundbreaking” pilot scheme, “for the very first time, migrants arriving via small boat will be detained and returned to France in short order”.
“In exchange for every return, a different individual will be allowed to come here via a safe route, controlled and legal, subject to strict security checks and only open to those who have not tried to enter the UK illegally.”
The Prime Minister said the pilot scheme, which will begin within weeks, “will show others trying to make the same journey that it will be in vain”.
Under the plan, those in France could express an interest to apply for asylum to the UK through an online platform and would then carry out the standard visa application process and checks.
Priority will be given to people from countries where they are most likely to be granted asylum as genuine refugees, who are most likely to be exploited by smuggling gangs and also asylum seekers who have connections to the UK.
It is not clear the criteria for deciding which migrants who arrive in the UK by small boat will be sent back to France, but it is understood the pilot will start with adults and will be based on operational factors.
New arrivals will be screened at Manston processing centre, in Kent, which is current procedure, before individuals determined to be suitable for the pilot and for detention, will be picked and held in an immigration removal centre.
Their removal is expected to be made on the grounds of inadmissibility, that they have arrived to the UK from a safe country where their case can be heard instead, because an agreement is in place with France.
Migrants will be able to claim exceptional circumstances against the decision.
It is expected more widely that appeals will not be successful as asylum seekers will be returned to France which is a signatory of the European Convention on Human Rights and has an established asylum system.
Explaining why the UK would take someone in exchange for a returned small boat migrant, Sir Keir said: “We accept genuine asylum seekers because it is right that we offer a haven to those in most dire need.
“But there is also something else, something more practical which is that we simply cannot solve a challenge like stopping the boats by acting alone and telling our allies that we won’t play ball.
“That is why today’s agreement is so important, because we will solve this, like so many of our problems, by working together.”
Sir Keir said it was “a real breakthrough in the way that we tackle the vile trade of people smuggling”.
The Prime Minister promised that the jobs migrants have been promised in the UK “will no longer exist because of the nationwide crackdown we’re delivering on illegal working, which is on a completely unprecedented scale”.
His comments follow Mr Macron’s warnings about the “pull factors” luring people into travelling through Europe to reach the northern French coast in the hope of reaching the UK.
Mr Macron said it was a “collaborative, co-operative and comprehensive plan”, beginning with work in the countries of origin of the migrants seeking to reach the UK.
He said that voters were “sold a lie” on Brexit and were told it would “make it possible to fight more effectively against illegal immigration”.
But because it left the UK without a returns agreement with the EU “it creates an incentive to make the crossing, the precise opposite of what Brexit had promised”.
Opposition politicians were scathing about the Prime Minister’s deal with Mr Macron.
Reform UK’s Nigel Farage, who spent the day on a boat in the English Channel watching migrants making the crossing, said: “This agreement is a humiliation for Brexit Britain.
“We have acted today as an EU member and bowed down to an arrogant French president.”
Shadow home secretary Chris Philp said: “Labour’s deal will only return one in every 17 illegal immigrants arriving.
“Allowing 94% of illegal immigrants to stay will make no difference whatsoever and have no deterrent effect.
“This is the latest catastrophic example that when Labour negotiates, the UK loses. “
