Wolfgang Metzeler-Kick, 49, began his protest in early March under the motto "Starving until you tell the truth", and has since been joined by three other activists.
He and his fellow protesters want Chancellor Olaf Scholz to acknowledge that "the climate catastrophe threatens the survival of human civilisation" and are calling for a "radical change of course" to reduce emissions in Europe's largest economy.
Metzeler-Kick, an environmental protection engineer and longtime activist, told AFP he was willing "to put (his) life in danger".
He stopped eating 62 days ago and wants to go further: "I'm going to intensify my hunger strike. As of tomorrow, I will temporarily stop drinking," he said.
Richard Cluse, a 57-year-old engineer, joined the protest 44 days ago, and Michael Winter, a 61-year-old biologist, joined after 22 days.
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A fourth activist, Adrien Lack, 34, joined on Tuesday with a placard reading: "I will only talk to the chancellor."
A doctor told a press conference on Tuesday that Winter was in a "very critical" condition and his body mass index had fallen below 16 kg/m2 -- considered severely underweight.
Climate activists have resorted to some eye-catching stunts to get their message across in Germany over the past two years.
Protesters from the radical group known as Letzte Generation ("Last Generation") have repeatedly sat down on busy roads and glued their hands to the tarmac.
Protesters have also thrown mashed potatoes over a Claude Monet painting in Potsdam and glued themselves to an exhibition of a dinosaur skeleton at Berlin's Natural History Museum.
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