HONG KONG—Hong Kong’s highest courtroom has denied bail to pro-democracy newspaper publisher Jimmy Lai, the most prominent figure going through prosecution underneath a sweeping national-protection law imposed by Beijing to silence opposition following a year of unrest.

The ruling signifies that Mr. Lai could perhaps commit the rest of his lifetime behind bars if convicted. The businessman and longtime critic of China’s Communist Occasion is charged with overseas collusion, which carries a optimum sentence of lifetime in prison. The determination also alerts that courts will set a large typical for granting bail to anyone charged underneath the national-protection law.

Mr. Lai, who is in his early 70s, is amongst a handful of defendants heading to demo underneath a law imposed very last year—and was the only a person so far that a decide experienced unveiled on bail pending demo. His situation is getting viewed closely in mainland China and in Hong Kong as a check of how the law will be enforced in the previous British colony, which has largely ruled itself and retained its U.K.-style lawful program since its return to China in 1997.

China’s point out-backed media reacted with outrage in December following a lessen-courtroom decide granted bail to Mr. Lai shortly following his arrest. Mr. Lai also faces a fraud cost. He denies the expenses.

China’s Communist Occasion mouthpiece The People’s Every day identified as Mr. Lai risky and castigated neighborhood courts for misreading the national-protection law, which sets rigid circumstances for bail. The column suggested mainland authorities may well invoke new powers underneath the law to pull Mr. Lai’s situation out of Hong Kong’s lawful program and consider him in the mainland.