The Grand Duke "will no longer participate in the legislative process, he will just sign the law to mark the completion of the procedure," Frieden said.
The constitutional revision is set to be put to parliament next Tuesday, according to Green party leader Francois Bausch.
A parliamentary source said that the 53-year-old Grand Duke, in power since 2000, had informed party leaders on Monday that he would not sign the law "for reasons of conscience," in a break from the sovereign's traditional political neutrality.
"Grand Duke Henri does not intend to sign the euthanasia bill," which would "therefore not enter into force," RTL radio echoed Tuesday.
Never in Luxembourg's history has the sovereign blocked a decision agreed in parliament.
The law, which would decriminalise certain types of euthanasia, is expected to be definitively adopted by the chamber at a second and final reading this month.
The Grand Duke, a member of the staunchly Catholic reigning family, should then normally "approve and promulgate" the law, by signing it within three months.
At its first parliamentary reading in February, the draft law was narrowly approved by 30 votes for and 26 votes against.
That vote was a defeat for the Christian Social People's party, Juncker's own party, whose members strongly opposed the move, fearing it would make euthanasia an everyday event.
It was approved thanks to support from Socialist deputies within the coalition government along with opposition liberals and Greens.
Source-AFP
SAV/SK