LUXEMBOURG, Luxembourg (AFP) — EU countries cannot jail illegal migrants just for crossing borders inside the passport-free Schengen area, the bloc’s top court ruled yesterday, in a new blow to efforts to tackle the migration crisis.

The European Court of Justice (ECJ) said European Union rules prevent the imprisonment of non-EU migrants who have illegally crossed a frontier, if they have not already been subject to deportation procedures.

The ruling came in the case of Ghanaian national Selina Affum, who was caught by French police at the Channel Tunnel while travelling by bus from Belgium to Britain using someone else’s passport.

French police placed her in custody for illegal entry to France, and then asked Belgium to readmit her.

Affum appealed against her detention, arguing that it was unlawful, and France’s highest appeals court referred the question to the ECJ.

The EU court ruled Tuesday that imprisoning illegal migrants was against the EU’s “return directive” — its laws on deporting migrants.

“The return directive prevents a national of a non-EU country, who has not yet been subject to the return procedure, being imprisoned solely because he or she has entered the territory of a member state illegally across an internal border of the Schengen area,” it said.

As Britain is not part of the Schengen area, the ruling also applied when a migrant “who is merely in transit on the territory of the member state concerned, is intercepted when leaving the Schengen area”.

France has a law saying that non-EU nationals can be jailed for a year if they enter French territory illegally, but the ECJ said EU law “precludes any legislation of a member state which lays down a sentence of imprisonment for an illegal stay”.

The passport-free zone of 26 European countries has come under severe pressure from the continent’s biggest migration crisis since World War II, as people flee war in Syria and elsewhere.

Faced with an influx of well over a million migrants and refugees over the past 18 months, many Schengen countries have brought back border controls that were dismantled a decade ago.

The court said that if a migrant refuses to leave voluntarily then EU states can only carry out a “forced removal using the least coercive measures possible”.

It said migrants could only be jailed if there was a risk of them evading deportation proceedings.

“It is only if there is a risk of the removal being compromised that the member state may keep the person concerned in detention, the duration of which may not, in any case, exceed 18 months,” the court said.

But it stressed that migrants could be jailed if they committed other crimes.