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.