Russia’s prison service on Monday gave Kremlin critic Alexei Navalny a last-minute ultimatum: Fly back from Germany at once and report at a Moscow office early on Tuesday morning, or be jailed if you return after that deadline.

Navalny, one of President Vladimir Putin’s leading critics, was airlifted to Germany for treatment in August after collapsing on a plane in what Germany and other Western nations say was an attempt to murder him with a Novichok nerve agent...