Hundreds of illegal immigrants returned from the United Kingdom to France under the “one in, one out” scheme have disappeared without a trace, The Guardian reports, citing human rights organizations. The missing include more than 40 individuals “of questionable age” who claimed to be unaccompanied minors.

The pact brokered last summer between London and Paris was promoted as a major achievement, allowing Britain to turn back illegal asylum seekers crossing the Channel while accepting asylum seekers with legitimate claims who had not attempted to enter illegally. However, the results of this scheme are considered quite limited: since it was signed in September last year, only about 1,400 migrants have been returned, compared with more than 32,000 illegal arrivals on British soil.

According to Maddie Harris, director of the Humans for Rights Network, among the returnees who have already disappeared were at least 41 people of questionable age. She recalled that the detention of unaccompanied minors in adult detention centers is illegal and they are categorically excluded from this agreement. On the other hand, the British Home Office has strongly rejected these allegations, insisting that all the deportees were adults and that no one of questionable age has been returned to France.

With the agreement set to expire in October, its future looks bleak. French Interior Minister Laurent Nunez has told his British counterpart, Shabana Mahmood, that Paris will not extend the bilateral pact. France wants to move away from this one-size-fits-all framework and is pushing for a broader agreement between the United Kingdom and the entire European Union, in which London would have to accept migrants from all 27 member states in exchange for those it sends back.