TLDR: You may be subject to an 80% surcharge on the tax due on the amounts held in the undeclared account or accounts, or a fixed fine per account when those amounts are not taxed. Transfers linked to an undeclared account may also be presumed to constitute taxable income, unless proven otherwise.
Financial penalties
Failure to declare a foreign account may result in an 80% surcharge on the tax due on the amounts held in the undeclared account or accounts.
When the amounts held in these accounts are not subject to any taxation, a fixed fine is provided for each undeclared account.
Errors and omissions in the declaration
Inaccuracies or omissions in the information that must appear in the account declaration are subject to a fine of €15 per omission or inaccuracy. The total amount of the fines applicable to documents that must be submitted simultaneously may not be less than €60 or more than €10,000, subject to the rules applicable to the document concerned.
Omissions in the declaration of changes to the account, as well as inaccuracies or omissions in the declarations referred to in the first paragraph of Article 1736 of the French General Tax Code, are subject to a fine of €150 per omission or inaccuracy. For information to be submitted simultaneously, the total amount of these fines may not exceed €10,000.
Presumption of taxable income
Amounts, securities or assets transferred to or from abroad through an undeclared account are presumed to constitute taxable income.
You may provide evidence to the contrary, particularly when the amounts are exempt, do not fall within the scope of the tax or correspond to income that has already been taxed.
Tax authority audits
During a desk audit or an accounting examination, the account statements may be used against the taxpayer. In particular, they may be used to apply the presumption of taxable income associated with transfers made through an undeclared account.