THE HAGUE (Reuters) - Judges at the International Criminal Court (ICC) ruled on Friday that Libya must hand over the son of slain dictator Muammar Gaddafi to the court, which wants to try him for alleged crimes committed during the revolution that toppled his father.

Libya had challenged the ICC's right to put Saif al-Islam Gaddafi on trial on the grounds that since Libya was planning its own proceedings, the ICC had no jurisdiction under the principle that it only intervenes if the local legal system is not up to the job.

But judges rejected Libya's claims, saying Libyan government lawyers had not proved their authorities were investigating the same crimes as the ICC's prosecutors.

They also questioned whether Libya had full custody over Saif al-Islam, something they would need if they were to try him. The British-educated 40-year-old is in the custody of authorities in the western mountain city of Zintan, where the writ of the central government runs weakly.

"Libya has not yet been able to secure the transfer of Mr Gaddafi from his place of detention under the custody of the Zintan militia into State authority," judges wrote, adding they were "not persuaded that this problem may be resolved in the near future."

Few expect Libya to surrender Saif al-Islam, even if Tripoli secures his transfer from Zintan. The Libyan government paid Maritania 200 million U.S. dollars, or 5 percent of the West African country's GDP, for the return of Gaddafi's co-accused Abdullah al-Senussi, defying an ICC arrest warrant.

Gaddafi, who is accused of orchestrating brutal reprisals against protesters during the short uprising that brought his father's iron-fisted 40-year rule to an end, is widely seen as an even more valuable symbol of his father's regime.

(Reporting by Thomas Escritt; editing by Andrew Roche)