Spain's El Mundo newspaper had reported that de Juana gave up his three-and-a-half month hunger strike after a supreme court ruling reduced his 12-year sentence for making terrorist threats.
But prison service officials confirmed on Saturday that de Juana is no longer receiving food after a tube used to force feed him was removed.
The supreme court ruling on February 12 reduced de Juana's sentence to three years. The prison service now has the option of freeing de Juana within a year, because of time he has already served.
But thousands of Eta victim group members and supporters of Spain's opposition Popular Party demonstrated in Madrid on Saturday to demand he serve the full three years of his sentence.
"He has been fed till now, but yesterday they took the tube out of him and now he isn't receiving any type of food because he's on hunger strike"
The Popular Party says de Juana is trying to blackmail the state.
Pictures of his emaciated body strapped to a hospital bed were published across the world earlier this month after a newspaper interview in which he asked to be released and called on the government to reopen peace talks with Eta.
The Spanish government abandoned the peace process after Eta claimed responsibility for a car bomb at Madrid International airport in December, killing two men in the group's first deadly attack in over three years.
Analysts say De Juana's death would spark new Basque violence and give the region a martyr.
De Juana's lawyer said his client's hunger strike will continue. "He has been fed till now, but yesterday [Friday] they took the tube out of him and now he isn't receiving any type of food because he's on hunger strike," Arantza Zulueta told Basque radio station Euskadi Irratia.
De Juana has already served 20 years for killing Spanish military and police officials during the 1980s as part of Eta's four-decade campaign for an independent Basque homeland.