Seif al-Islam, who has been in a five-and-half-year legal tug-of-war between the ICC and Tripoli, is wanted by the tribunal in The Hague to face charges of crimes against humanity relating to the bloody repression of the 2011 uprising that toppled his father.
Seif, 44, has been held in the northern hilltop stronghold of Zintan since his arrest in November 2011.
Last year, in a move heavily criticised by the UN, he was sentenced to death by a Libyan court for trying to put down the deadly revolt that saw Moamer Kadhafi's 40-year rule end in his own murder.
The new lawyers told journalists in The Hague that they will ask the ICC's judges to scrap the case, basing their argument on the so-called principle of "double jeopardy".
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"The reality is that a trial has taken place. He has been tried and convicted in Libya. It's a clear principle of law that one cannot be tried twice for the same offence," veteran defence lawyer Karim Khan told a press conference.
"The court will receive a filing from us, the lawyers of Seif al-Islam in due course, seeking to declare the case inadmissible," Khan said.
Zaidy last met with Seif in prison in late 2015 and said he is in regular contact and receiving instructions from him. "Healthwise he is rather good," he said, but Seif had been thinking a lot about the situation in Libya.
Despite the death sentence, Seif may benefit from a proposed general amnesty law in Libya that his family had been assured "will be applicable to all Libyan nationals without exception," Zaidy said.
Seif and eight others were sentenced to death by a Tripoli court in July last year. They included Kadhafi's former intelligence chief Abdullah Senussi, who saw the ICC drop a similar case against him.
His legal team also insisted he is being held a Libyan government jail run by the justice ministry and that rumours he was in the hands of Zintani militias were "totally groundless". In the past the militias refused to hand him over to Tripoli and the ICC.