Interior Minister Gérard Collomb welcomed the judgement in a statement, saying that the refusal to extend exemptions from prosecution for aiding illegal entry "validates the government's policies".
But the lawyer for farmer Cédric Herrou, a solidarity campaigner who brought the case so as to stop prosecutions of activists who help migrants, also hailed an "immense victory".
"The principle of fraternity has been recognised," Patrice Spinosi told the media. But he regretted that the question of aiding entry was not covered by the ruling.
Asylum law reform
Herrou and another campaigner appealed against two clauses of France's recently amended asylum and immigration law.
One allows for sentences of up to five years in jail and a 30,000-euro fine for helping "illegal entry, circulation and sojourn".
The other, amended during the recent parliamentary debates on the law in the light of complaints that it made solidarity a crime, rules out prosecution if the case concerns relatives or individuals "when the relevant act has given rise to no direct or indirect material reward".
Permissible help would mean "providing legal advice or food, lodging or medical care... or all other help that aims to preserve the dignity or physical well-being".
Herrou and his team found the formulation "too vague", arguing that it would leave activists open to prosecution as if they were people traffickers.
“It was not clear whether citizens or NGOs when helping migrants would be prosecuted,” Jules Bejot, of Brussels-based thinktank Migration Policy Group told RFI.
“Today what the constitutional court has said is that when helping people on a humanitarian basis and without any remuneration there should never be any criminalisation. But the ambiguity remains on one point which is helping people to enter onto French soil.”
Liberty, equality, fraternity
Collomb at the time criticised "an extremely dangerous category of totally irresponsible people ... who are calling for the abolition of borders".
Citing the French republic's motto of "Liberty, equality, fraternity", the Constitutional Council found that, by "banning all help provided to an undocumented foreigner", the law had not maintained balance between the "principle of fraternity" and "preserving public order".
"The freedom to help another, for humanitarian reasons, follows from the principle of fraternity, without consideration of the legality of their presence on the national territory," it declared.
Immunity from prosecution should apply to "all assistance provided with a humanitarian aim", it added.
The decision to recognise fraternity as a constitutional value should serve as jurisprudence, the council's president, former foreign affairs minister Laurent Fabius, said on Friday.
“The constitutional court has asked parliament to get rid of the amendments which enable the prosecution of Cédric Herrou and others like him,” Bejot commented. “We hope that now parliament will review the law and bring it into line with the principle of fraternity which is the basis of our constitution. This is really significant because what it means is Cédric Herrou and others should have the freedom to help people in need.”