Aug. 7 (UPI) -- A landmark Anglo-French "one-in, one-out" migrant agreement saw the first small boat arrivals on the British coast taken into custody in preparation for being returned to France, the government said Thursday.

The detentions got underway on Wednesday with migrants who had crossed the Channel "illegally" held in secure immigration centers pending their removal to France, which was expected to take place in a matter of weeks, according to a Home Office news release.

It pledged full transparency, saying detainees would be briefed on the process for returning them to France and kept updated on their progress through the system on an individual basis.

For each migrant sent back, Britain will take in one pre-approved to claim asylum who has not previously attempted to enter the country and who has completed a formal application and security clearance process in France that is only open to those with a passport or identity document.

Pre-checked individuals, or family groups, will then journey safely from France via scheduled rail, ferry or airline services.