The 78-year-old founder of the now-defunct Apple Daily newspaper was found guilty in February on charges of foreign collusion and seditious publication.

His sentence is the harshest penalty doled out so far under a national security law imposed by Beijing on Hong Kong following widespread pro-democracy protests.

"We have clear and definitive instructions (from Lai) not to lodge an appeal against conviction or sentence," his legal team told AFP, adding that they would not provide an explanation for the decision.

Lai's conviction received international condemnation at the time, with rights groups condemning his punishment as "effectively a death sentence" and a symbol of the city's shrivelling press freedoms.

His son, Sebastien Lai, said the "draconian prison sentence is devastating for our family and life-threatening for my father".

UN rights chief Volker Turk has called for the verdict to be "promptly quashed as incompatible with international law".

US President Donald Trump, who is expected to visit China in late March, has urged China's leader Xi Jinping to free Lai.

China rejected the criticism, with Beijing saying after the sentencing it lodged "solemn representations" with nations that condemned the sentence.

Hong Kong's leader John Lee lauded the "severe" prison term as a demonstration of the city's rule of law.

Eight other defendants, including six Apple Daily executives, were handed jail sentences of up to 10 years in the same case. All except Lai had pleaded guilty.