Migrants cannot be imprisoned just because they have entered a country illegally, Europe’s highest court has ruled, in a potential blow to British, Dutch and French police attempting to prevent people smuggling over the Channel.

France had no right to detain a Ghanaian woman who tried to sneak into Britain by travelling on the Channel Tunnel with someone else’s passport, the European Court of Justice ruled.

Doing so breached the EU Return Directive which say illegal migrants must be invited to go home voluntarily first, within a window of up to a month.