The first migrant has been deported back to France under Britain’s "one in, one out" deal, the Home Office has confirmed.
A man from India who arrived in Britain by small boat in August was put on board an Air France plane to Paris on Thursday morning.
Further flights are due to take place this week and next week despite legal challenges, the Government said.
Home Secretary Shabana Mahmood said: “This is an important first step to securing our borders. It sends a message to people crossing in small boats: if you enter the UK illegally, we will seek to remove you.
“I will continue to challenge any last-minute, vexatious attempts to frustrate a removal in the courts. The UK will always play its part in helping those genuinely fleeing persecution, but this must be done through safe, legal, and managed routes – not dangerous crossings.”
It comes as the Government faced fresh pressure over sending asylum seekers who arrive in small boats back across the Channel when planned flights for removals were cancelled earlier this week.
Under the “one in, one out” pilot scheme, Britain will detain some of the people who cross the Channel and send them back, in return taking an asylum seeker in France who can show they have family connections in the UK.
More than 31,000 migrants have made the dangerous journey in small boats so far this year.
The Home Office lodged an appeal on Thursday against a High Court decision temporarily blocking the deportation of an Eritrean man under the returns agreement.
On Tuesday, the court had granted the man a "short period of interim relief" and given him 14 days to make representations to support his claim that he was a victim of modern slavery.
Ms Mahmood is carrying out an urgent review of the Modern Slavery Act to assess whether it is open to abuse.
She said: "Migrants suddenly deciding that they are a modern slave on the eve of their removal, having never made such a claim before, make a mockery of our laws and this country's generosity.
"I will fight to end vexatious, last-minute claims. I will robustly defend the British public's priorities in any court. And I will do whatever it takes to secure our border."
The Independent Anti-Slavery Commissioner Eleanor Lyons condemned the Home Secretary's comments.
She said Ms Mahmood's words "have a real-life impact on victims of exploitation, who may now be more scared to come forward and talk about what's happened to them".
Senior Treasury minister James Murray rejected suggestions that Labour had forgotten its values as he faced questions about the Government response.
"What's driving this is what's important to the British people," the Chief Secretary to the Treasury told Sky News on Thursday.
"As a Government, we're responding to what's important to people in the UK, and people have said, and people are right to feel angry about the level of illegal migration.
"People are right to feel they want hotels to close. People are right to feel they want the Government to do more on this, and that's exactly why we're doing more on this."
