The grand duke said his conscience prevented him signing the law
Luxembourg's Grand Duke Henri is set to have his powers trimmed by parliament in a row over euthanasia legislation.
The grand duke signalled that he would not sign the euthanasia bill into law even though the Chamber of Deputies approved it in February.
His stance has prompted PM Jean-Claude Juncker to push for the constitution to be re-worded so that the duke merely signs laws, rather than approving them.
The revision is expected to happen before the end of the year.
"That means he will only technically enact laws," said Mr Juncker, quoted by Reuters news agency.
Grand duke's conscience
Luxembourg is a constitutional monarchy in which the Grand Duke holds executive power and bills only become law with his signature.
The law on euthanasia and assisted suicide would allow those with incurable conditions to die if they asked repeatedly to do so and had the consent of two doctors and a panel of experts.
It was due to have a final reading later this month, but Mr Juncker said the constitutional amendment - requiring a two-thirds majority in parliament - would happen before then.
Grand Duke Henri, 53, said "reasons of conscience" prevented him from signing the bill, AFP news agency reported.
Mr Juncker, whose Christian Social Party had opposed the bill, said: "I understand the grand duke's problems of conscience.
"But I believe that if the parliament votes in a law, it must be brought into force."
Mr Juncker's party was defeated in the vote when Socialist deputies within the coalition government joined opposition Liberals and Greens to back the legalisation of euthanasia.