Jimmy Lai, the billionaire Hong Kong media tycoon, has been sentenced to almost six years in prison for fraud.
A judge said Lai, found guilty of illegally subletting office space in October, felt no remorse.
The 75-year-old recently served time for taking part in a banned vigil last year for victims of China’s 1989 Tiananmen Square massacre.
His son stressed it was “disappointing” the UK was not doing more to help Lai, who holds British nationality.
In a separate statement, Sebastian Lai said his father done nothing wrong and had already spent time in prison for “standing up for human rights”.
Lai faces the prospect of life behind bars due to a separate trial on national security charges.
Those proceedings had been due to begin on 1 December but were postponed at the request of Hong Kong leader John Lee Ka-chiu.
China has enforced its wide-ranging national security law on the city of Hong Kong, making it easier to prosecute protesters.
The law has led to the arrests of many prominent democracy activists.
Hong Kong, a former British colony, returned to China’s rule in 1997.
As well as being sentenced to five years and nine months, Lai was fined 2m Hong Kong dollars (£209,535).
The leader of Mr Lai’s international legal team, Caoilfhionn Gallagher KC, said Lai was the victim of “lawfare”.
She said the multiple cases were “all designed to silence and discredit [Lai] and send a clear message to others that they should not dare to criticise the Chinese or Hong Kong authorities”.
Hong Kong accused of silencing critics with legal system
By Danny Vincent in Hong Kong
Jimmy Lai has been in custody since December 2020. On Thursday, he celebrated his 75th birthday in prison. Under the national security law he could face the rest of his life in jail.
He had long been considered the number one target of the Beijing-imposed law. But that case has not even begun.
Today’s judge told the court that the sentence of five years and nine months was for a “simple case of fraud”.
But critics say the legal system has become a weapon to silence the political opposition.
Some say that the introduction of the national security law has spelt the end of the rule of law in Hong Kong.