The former Hong Kong leader Donald Tsang has been sentenced to 20 months in jail for misconduct in public office, making him the most senior city official to be imprisoned in a ruling some said reaffirmed the territory’s vaunted rule of law.

The sentencing on Wednesday brings an ignominious end to what had been a long and stellar career for Tsang before and after the 1997 handover to Chinese control, service that saw him knighted by the outgoing British colonial rulers.

“Never in my judicial career have I seen a man falling from such a height,” said high court justice Andrew Chan in passing sentence.

Tsang, 72, wearing one of his trademark bow ties, was escorted in handcuffs to the court from hospital where he had been staying since Monday night after experiencing breathing difficulties and chest pains.

The devout Catholic appeared stoic, occasionally closing his eyes as the judge spoke.

Many establishment Hong Kong figures, including top former officials and some leading opposition democrats, had written letters vouching for Tsang’s good character and longstanding public service.

Chan said the seriousness of the offence lay in Tsang’s high position as a person of integrity who had breached public trust. He reduced the sentence by 10 months, saying “it was indisputable that the defendant has dedicated himself to public service in the past 40-odd years”.

Hong Kong returned to China under a “one country, two systems” agreement that ensures its freedoms, including a separate legal system. Its spartan British-built prisons demand strict routines, including light work duties, and offer no special treatment to wealthy or powerful inmates.

A nine-person jury on Friday found Tsang guilty of a charge of misconduct in public office. He had deliberately concealed private rental negotiations with property tycoon Bill Wong Cho-bau while his cabinet discussed and approved a digital broadcasting licence for a now-defunct radio company, Wave Media, in which Wong was a major shareholder.

This offence had occurred at the twilight of Tsang’s career, just before retiring in 2012. Tsang was acquitted of a second misconduct charge.

In a regular column published in the AM730 newspaper before sentence was passed, Tsang said working in the government for 45 years had been the biggest honour of his life.

“In life, a lot of things are out of our control. But serving Hong Kong was my choice. No matter what the result of the trial is, I have no regrets.”

His conviction adds to a number of scandals ensnaring powerful officials that have marred the city’s reputation as a relatively corruption-free society guarded by a powerful and independent anti-graft agency.

His right-hand man, Rafael Hui, who worked under him for two years as the city’s second highest-ranking official, was sentenced to seven and a half years in jail in late 2014 for receiving bribes from a billionaire tycoon at the head of Sun Hung Kai, one of Asia’s largest property developers.