A senior leader of the second Palestinian uprising also called on Thursday for the release of Johnston, describing him as "a friend of the Palestinian people".
Marwan Barghouti, the West Bank leader of Fatah, made the appeal from the Israeli prison where has been held for nearly three years.
"From my prison cell and in the name of the 10,000 Palestinian prisoners I call for the immediate release of journalist Alan Johnston, a friend of the Palestinian people," he said.
He rejected "kidnappings and aggressions that enormously harm Palestinian interests and our national struggle".
Johnston, 44, was seized at gunpoint as he drove home from work in Gaza City on March 12.
One of the few western reporters to have both lived and worked in Gaza, he has become the longest-held kidnapped westerner in the territory.
On Sunday, an Islamist group called the Kataeb al-Jihad al-Tawheed (The Brigades of Holy War and Unity) claimed in a statement to have killed Johnston.
However, Palestinian authorities say there is no proof that the claim by the little-known group was true.