Libyan National Congress spokesman, Omar Hamdan, said the 28-year-old died 'in battle' but gave no further details, the Daily Mail reported.
The body of the seventh son of the late Libyan leader is said to have been found yesterday after fierce fighting between pro-Gaddafi forces and militias allied to the Libyan government in the town of Bani Walid, the report said.
However, there are conflicting accounts about whether Khamis Gaddafi was killed during conflict or fatally wounded and later died.
"The seventh and youngest son of the Libyan slain strongman Muammar al-Qaddafi has been declared dead after his capture on Saturday," Al Arabiya news channel quoted its sources as saying.
Khamis al-Qaddafi, who was a military commander in charge of the Khamis Brigade of the Libyan Army, was severely wounded when arrested, it said.
Hiteen Brigade, belonging to the Military Revolutionary Council in the northwestern city of Misrata, arrested Qaddafi's son, exactly a year after the dictator's death, the sources added.
There have been several reports in the past that he has been killed, including during a NATO airstrike in August 2011.
Khamis, who studied at a Russian military academy, was one of the most hardline of Gaddafi's sons and formed the 32nd Brigade to serve his father.
His forces have been blamed for committing murder, rape and torture, the Mail said.
Government forces claimed 13 people were killed and 121 wounded on a fourth day of bitter clashes for control of Bani Walid.
Reports of his death brought scenes of wild celebrations in Misrata, the city which had been under siege for six months by Khamis's 32nd Brigade.
It is believed his body was being brought to the city, the third largest in Libya, as his father's corpse had been after he was killed by rebels on October 20 last year.
Khamis never married and with his older brother, Moatasen, proved one of the enforcers of the Gaddafi family.
His other brother, Saif Al Islam, is currently in a Libyan jail on war crimes charges.
