The UK and France will have to stop roughly 85 per cent of attempted migrant crossings to dismantle the business model of people smugglers, officials believe.
Under the new cross-Channel treaty, the first migrants arriving on British shores will be detained by the end of the week, the Home Office said.
The "one-in, one-out" pilot scheme came into force on Tuesday and was approved by the European Commission.
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Beds have been freed up at immigration removal centres, while the Border Force are preparing to select those illegal migrants to be removed.
The Home Office confirmed that any adult migrant, arriving on small boats, will face being sent back.
An equal number of legitimate asylum seekers in France, who have never attempted the trip across the Channel, will move the other way.
The pilot scheme will run for almost a year, however, it has received backlash with it understood that about 50 migrants will be deported each week.
The figure equates to around one-in-17 of all arrivals if crossings were to continue at the current pace.
Migrants caught crossing the Channel can expect detention in the coming days | REUTERSA senior Home Office source told The Times that modelling by officials estimated measures to fight against the crossings would need to prevent up to 85 per cent of all attempted crossings so that people smugglers' business model could be dismantled.
However, a source close to Home Secretary Yvette Cooper told The Times they rejected the modelling.
They said it was outdated and "doesn't reflect the full range of policies the new government has put in place to tackle this issue".
"From our work with the French to stop boats leaving in the first place, to the massive expansion in law enforcement and the significant increase in deportations," the source said.
A source close to Yvette Cooper rejected the modelling
| GETTYThey further said Labour planned to make it more expensive for migrant crossings to deter people from making the trip.
The pilot scheme with France is one of the many policies adopted by Prime Minister Sir Keir Starmer's Government, with deals also recently made with Germany.
However, ever since the small boats crisis started in 2018, crossings have continued to increase to hit its highest level yet.
More than 25,000 illegal migrants have already crossed the Channel this year, 49 per cent higher than during the same period last year.
Emmanuel Macron and Keir Starmer announced the one in, one out migrant deal | PAIn his bid to "smash the gangs", Starmer said the new French deal as an unprecedented scheme which would target the people smugglers' business model.
The Prime Minister claimed: "This Government has been fixing the foundations of the broken asylum system we inherited and today we send a clear message — if you come here illegally on a small boat you will face being sent back to France."
Starmer added that it was the product of "months of grown-up diplomacy delivering real results for British people".
"We (are brokering) deals no Government has been able to achieve and strike at the heart of these vile gangs' business model," Starmer said.
Migrants arrive in Kent | PA"The days of gimmicks and broken promises are over - we will restore order to our borders with the seriousness and competence the British people deserve."
It is understood France will be given a veto power on which migrants the UK sends back.
Britain will then have to accept asylum seekers already in France in exchange.
Critics have also warned that the deal with France opens up a major legal loophole, with particular concerns being raised about how the UK will prove "unfounded claims" for asylum.