Migrants who arrived in Britain on small boats this week could be deported back to France within weeks.
Multiple people have been detained under the new "one in, one out" deal after crossing the English Channel, the Prime Minister announced on Thursday.
Sir Keir Starmer said: "We have detained the first illegal migrants under our new deal before returning them to France.
"If you break the law to enter this country, you will face being sent back.
"When I say I will stop at nothing to secure our borders, I mean it."
Home Secretary Yvette Cooper said the Government was prepared to defend itself against legal challenges if people seek to avoid being sent back across the English Channel.
The first detentions came as migrants arrived in Dover on Wednesday, the day the pilot scheme came into force.
Under the plan, adults arriving on a small boat can be detained and returned to France for the first time in exchange for other asylum seekers who have a link to the UK.
It means the approved asylum seeker will be brought to Britain under a safe route.
UK officials aim to make referrals for returns within three days of a person’s arrival by small boat, while French authorities will respond within 14 days.
Adults and families in France are able to express an interest in coming to the UK via an online platform set up by the Home Office.
They will have to meet suitability criteria, including standard visa applications and security checks.
If accepted, they will be given three months in Britain to claim asylum or apply for a visa, and will be subject to the same rules that means asylum seekers are not allowed to work, study or have access to benefits.
The deal comes as ministers grapple to crack down on smuggling gangs amid a record number of people making the dangerous crossing.
The Home Office said detentions began for those who arrived on Wednesday afternoon and they will be held in immigration removal centres until they are returned to France.
Ms Cooper said: "Yesterday, under the terms of this groundbreaking new treaty, the first group of people to cross the Channel were detained after their arrival at Western Jet Foil and will now be held in detention until they can be returned to France.
"That sends a message to every migrant currently thinking of paying organised crime gangs to go to the UK that they will be risking their lives and throwing away their money if they get into a small boat."
