Euronext chief executive Stephane Boujnah said the fusion would create a company 10 times larger than his in second place.
“This kind of dominance can lead to abuse of power,” the Frenchman said in an interview with Frankfurter Allgemeine.
Boujnah also raised concerns about the London location of the merged group’s holding company.
The UK and German companies officially submitted their merger to European Union competition regulators at the end of last month.
The EU Commission is expected to announce the launch of an in-depth, phase two, investigation towards the end of this month.
In addition to Euronext, the governments of France, Netherlands, Belgium and Portugal have all voiced opposition to the merger.
In an interview with City A.M. in July, Boujnah said his company was monitoring the LSE deal’s progress and indicated that, were it go ahead, he would be watching to see “whether some opportunities can come out”.
Euronext has been linked with parts of the LSE group, such as Clearnet and Borsa Italiana.