A former Singapore policeman once portrayed as a model officer was sentenced to hang Friday for the gruesome double murder of a businessman and his son in a case that has shocked the city-state.

Ex-Senior Staff Sergeant Iskandar Rahmat, 36, was found guilty of murdering car workshop owner Tan Boon Sin, 67, and his son Tan Chee Heong, 42, inside their home in July 2013 during a robbery that went awry.

The two bore multiple stab wounds and the younger man's body was dragged about a kilometre (mile) after it got caught under his father's car that Iskandar used as a getaway vehicle, leaving a trail of blood on a busy road as other motorists watched in horror.

The case shocked the city-state, one of Asia's safest cities, where such murders and the involvement of law enforcers in crimes are rare.

Defence lawyers said the killings were not premeditated and argued for a conviction on a lesser charge that did not mandate the death penalty.

However, High Court Judge Tay Yong Kwang disagreed.

"It follows from my findings that the accused caused the death of (the father and son) by attacking them cruelly and relentlessly with the clear intention of causing death," he said.

Defence lawyer Shashi Nathan told reporters he will file an appeal.

Iskandar was once portrayed as a model officer on the Singapore Police Force website, but he had fallen into debt and was facing possible expulsion from the department.