BNP Paribas bank, accused by non-governmental organisations of complicity over a transfer of $1.3 million to an arms dealer, said the move was expected and not a surprise.
"This in no way constitutes a new development," a BNP spokeswoman said.
The full-scale inquiry follows a preliminary investigation that began earlier this year, when three non-government organisations filed a complaint on the matter.
In France, the opening of a full-scale inquiry does not imply guilt and does not automatically lead to a trial. It does imply that investigators believe the complaint or information gleaned during initial inquiries merits further investigation.