An online report has said the more than "three-quarters" of Bettel's postgraduate dissertation was written without proper citation.
Luxembourg Prime Minister Xavier Bettel has admitted allegations of plagiarism over his university dissertation.
PM Bettel acknowledged that he "should have done differently" after the revelations first appeared online.
The daily Reporter.lu made accusations about the thesis Bettwel wrote in 1999 while studying a postgraduate in public law and political science at the University of Nany in France.
According to the online media, more than "three-quarters" of the thesis were written without proper academic citation.
Bettel said in a statement that he had written the work "more than 20 years ago ... to the best of my knowledge and belief at the time".
"From today's perspective, I recognise that I should have done it differently, perhaps I should have done it differently," he added.
The Luxembourg PM said he would now leave it to the university -- now called the University of Lorraine -- to "assess" whether the dissertation met the criteria when it was written.
"The institution takes breaches of scientific integrity seriously and an investigation will be carried out into the content of the dissertation," the University of Lorraine reacted in a statement.
"The possible sanctions that the institution may have to take will depend on the conclusions of the investigation."
The university added that at the time, institutions "were not equipped with the current anti-plagiarism software".
Bettel -- who has been prime minister since 2013 -- said he would "naturally" accept the university's decision even if his qualification was withdrawn.
The 56-page DEA dissertation in question was titled "Towards a possible reform of voting systems in the European Parliament elections".
Reporter.lu says that 96% of the work consists of copying excerpts from two books, one newspaper article, and four websites.
It said only “a few paragraphs in the introduction” and “an equally short conclusion” had not been copied wholesale.
In neighbouring Germany, several ministers have resigned in recent years following accusations of plagiarism, including former family minister Franziska Giffey last May.