The member of parliament for Saône-et-Loire was speaking on radio station Europe 1 on Tuesday morning and expressed some of the anger that many French feel about the country's recent loss of its AAA rating from Standard & Poor's.
"For me, the rating agencies are charlatans," he said. He complained they were also "at the origin of the global subprime crisis because they gave these toxic products a AAA rating."
"And now they're making money on the back of European states," he said.
Montebourg proposed that use of the ratings agencies should be banned in France, which would "deprive them of revenues."
He also suggested a publicly-financed European auditor would be better placed to rate sovereign debt than the privately-run ratings agencies.
Montebourg performed well in the October elections to choose the Socialist candidate who will likely challenge president Sarkozy in April's presidential elections.
He campaigned on a left-wing platform of protectionism and more state intervention, polling 17 percent of the votes. François Hollande ultimately won the contest.
He has published a book arguing for "deglobalization," a process that would reverse the trend of shifting production to low cost regions around the world.
The Economist magazine attacked his views as "patent nonsense" in an editorial in August which also described the French Socialists as "Europe's most unreconstructed" left-wing party.
Montebourg has been more prominent since his strong performance in the vote and is likely to be given a ministerial position in a future government, should the Socialists win the spring vote.