A group of Channel migrants are trying to dodge deportation to France by launching legal action against Shabana Mahmood's modern slavery reforms.
The Home Secretary vowed to change the laws after asylum seekers exploited loopholes to remain in the UK.
But a fresh group of 16 will today argue Ms Mahmood's reforms are unlawful.
The cases are expected to focus on whether French obligations under key human rights treaties apply to foreign nationals and whether a key clause designed to limit appeals is legal.
Home Office sources have said the department will “fight” the claims, which will be heard at the Court of Appeal.
Shadow Home Secretary Chris Philp said: “The government's so-called one in one out policy was always a gimmick – just like their ridiculous claim to smash the gangs.
“Last year they let in 41,000 illegal Channel immigrants and only removed 300 to France.
“That's no deterrent. No wonder illegal Channel crossings are up 42% since the election.
“And now even this gimmick faces legal challenge.
“We need to leave the ECHR and the modern slavery treaty – then every illegal immigrant can be deported within a week of arrival with no messing around.”
Home Secretary Shabana Mahmood last week revealed just 305 migrants have been returned to France since the scheme became operational in August.
And within days, migrants began lodging modern slavery claims to avoid being deported.
Ms Mahmood responded by overhauling the modern slavery laws, with much greater scrutiny placed on claims made at the 11th hour.
The Home Office is also removing the “reconsideration” mechanism for all countries that have signed the European Convention on Human Rights and the Council of Europe Convention on Action Against Trafficking in Human Beings.
Minister for Border Security and Asylum Alex Norris said: “Last-minute modern slavery claims must never be used to prevent the removal of illegal migrants.
“The Government will fight any legal attempt in the courts to frustrate their removal or deportation. These cases are exactly why we are reforming our laws to stop these last-minute claims and restore order and control to our border.”