David Cameron said that he has received assurances from Libyan leaders that captured fugitive Saif al-Islam will be tried in line with international standards.
Britain will offer "every assistance" to Libya's government to ensure Muammar Gaddafi's son is brought to justice over his role in the "barbaric" reign of terror, the Prime Minister added.
Al-Islam was seized in southern Libya with two aides, who were trying to smuggle him out to neighbouring Niger, officials confirmed.
Mr Cameron said: "The Libyan government's announcement of Seif al-Islam's arrest shows we are near the end of the final chapter of the Gaddafi regime.
"It is a great achievement for the Libyan people and must now become a victory for international justice too.
"He could have contributed to a more open and decent future for his country, but instead chose to lead a bloody and barbaric campaign against his own people. The fate of the Gaddafis should act as a warning to brutal dictators everywhere.
"Britain will offer every assistance to the Libyan government and the International Criminal Court to bring him to face full accountability and justice for what he has done. The Libyan government has told us again that he will receive a trial in line with international standards, and it is important that this happens."
Al-Islam was flown to the city of Zintan in northern Libya and is said to have told international reporters on the plane that he was in good health, saying that injuries on his right hand were caused by a Nato air strike a month ago.
Foreign Secretary William Hague said the arrest would allow Libya to "move on".
He said: "I welcome the arrest of Seif al-Islam. This represents another significant step forward in the transition to a new, democratic Libya. He must now be held to account for his actions, and face trial on the charges brought against him, including by the International Criminal Court."
