Khalida Jarrar was arrested on July 2 for being a senior member in the Popular Front for the Liberation of Palestine (PFLP), a movement considered a terrorist organisation by Israel, the United States and the European Union.
She had been released from prison only a year before.
Details of the accusations against her are secret, as is customary with Israeli administrative detention orders, which allow detention without trial for renewable six-month periods.
On December 24, the detention was extended by another six months "after security personnel found she still poses a substantial threat," the Israeli army said.
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"The decision to hold Jarrar under administrative detention was made as a last resort, after the military official who signed the arrest warrant was displayed confidential information that clearly substantiates the threat posed by Jarrar," the army said in a statement.
Many PFLP leaders are in custody and Jarrar has been jailed multiple times.
The 54-year-old had only been released in June 2016 after 14 months in an Israeli jail for allegedly encouraging attacks against Israelis.
But the system has been criticised by Palestinians, human rights groups and members of the international community who say Israel abuses the measure.
The Addameer rights group says around 6,150 Palestinians are currently in Israeli jails, including around 450 in administrative detention.
Jarrar is the 11th member of the Palestinian parliament currently in jail, according to the Palestinian Prisoners' Club NGO.
According to the Prisoners' Club, Palestinian lawmaker Nasser Abdel Jawwad of Hamas was arrested this week in Salfit in the occupied West Bank. The military had not immediately responded to a request for confirmation.
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