Yvette Cooper refused to say how many migrants have been detained under Labour’s new returns deal with France. The first small boat arrivals were detained yesterday as Sir Keir Starmer’s “one in, one out” agreement came into force.

But the Home Secretary would not give details about how many migrants were being held in immigration removal centres. Pressed for a figure, she said: “We will provide regular updates for the public at every stage. That’s why I’m confirming today that the pilot has now started and the first migrants who arrived on small boats are now in detention.

“The transfers to immigration removal centres are under way as we speak, so we won’t provide operational details at this point that criminal gangs can simply use and exploit.

“But no one should be in any doubt: anyone who arrives from now on is eligible for immediate detention and return.

“This pilot will now build because we are determined that no one should be making this journey and this is a vile trade in people.”

Asked how soon migrants would be deported, Ms Cooper added: “The pilot has now begun, so the first migrants who have arrived on the small boats are now in detention.

“We will then swiftly make the referrals to France and that process will now start to be able to return people to France.

“It’s the beginning of the pilot and it will build as well over time, but we’re also clear that France is a safe country, so we will robustly defend against any legal challenge that people try.

“We do expect for people to start being returned in a matter of weeks.”

Under the pilot scheme, adults arriving on small boats can be detained and sent back to France for the first time.

This is in exchange for an approved asylum seeker in France to be brought to the UK under a safe route.

There have been reports that 50 people per week could be returned, compared with the weekly average of more than 800 people currently making the crossing.

Tory Shadow Home Secretary Chris Philp said: “Keir Starmer’s promise last year to ‘smash the gangs’ has turned out to be nothing more than a gimmick that didn’t work, and this is just the same.

“They are detaining a token handful of arrivals and in return we accept unvetted migrants from France. The whole thing is riddled with loopholes, opt-outs and legal escape routes that will make removals near-impossible.

“94% of illegal arrivals will still remain in the UK under this deal. How exactly is that supposed to deter anyone?”

Ministers have rejected criticism that the returns deal leaves open a loophole for human rights laws to be exploited for migrants to avoid deportation.

It comes as more than 25,000 people have made the dangerous journey so far this year.