PARIS—France will slash the number of visas available to nationals from Maghreb countries because of their governments’ refusal to take back illegal immigrants sent home by the French authorities, government spokesman Gabriel Attal said on Tuesday. Immigration is becoming a key campaign issue for the French presidential election set for April next year, with conservative parties challenging President Emmanuel Macron’s policies. Macron has not yet said whether he will stand for re-election. Attal said the French government would halve the number of visas available to nationals from Algeria and Morocco and reduce those for Tunisians by almost a third. “It is a decision that is made necessary as these countries do not accept back nationals whom we do not want and cannot keep in France,” he told ‘French Europe 1’ radio. Morocco’s foreign minister Nasser Bourita told reporters in Rabat the decision was “unjustified” and “does not reflect the reality …