In an urgent filing to the Hague-based court, British lawyer John Jones asked appeals judges to reject Libya's request to suspend an order that Tripoli surrender Seif al-Islam to the court.
Libyan authorities are appealing the international court's right to try Gadhafi's one-time heir apparent, saying that he should face justice at home, but the international court says Tripoli cannot give him a fair trial.
Jones says that Seif al-Islam could be executed in Libya before the appeal is completed if he is not handed over to the court.
"The possible implementation of the death penalty in domestic proceedings would also create a grievous and irremediable consequence for Mr. Gaddafi, and completely undermine the ability of the Appeals Chamber to render a determination on the appeal," Jones wrote.
Libyan prosecutors said Monday that Seif al-Islam, Gadhafi-era spy chief Abdullah al-Senoussi—who also is wanted by the ICC—and ex-premier al-Baghdadi al-Mahmoudi, along with ex-spokesman Milad Daman, will be tried in August for crimes committed during Gadhafi's 42-year rule and the eight-month civil war that deposed him.
Jones said the announcement "could only be construed as blatant disregard" for Libya's obligations to the court.
Seif al-Islam is being held by a militia in the Libyan town of Zintan.
With no national army or police in place since the fall of Gadhafi's regime, successive governments have been too weak to either secure Seif al-Islam's imprisonment in the capital, Tripoli, or put pressure on his captors to hand him over to the government. Gadhafi's son is also being tried on separate charges of harming state security.