The lockdown in place since March 17 is widely expected to be extended into April, and the government has pledged 45 billion euros (49 billion dollars) in loan guarantees and other relief for firms.
Insee said a one-month lockdown could slash annual gross domestic product by three percentage points, but cautioned that it lacked sufficient data to provide a firm estimate on French economic growth in the coming months.
The government has already said it expects GDP to contract one per cent this year, abandoning the 1.3 per cent growth target of just a few weeks ago, along with a huge jump in the budget deficit.
Services, heavy industry and construction are all taking big hits, Insee said, as factories are shut down and only essential businesses like supermarkets or pharmacies remain open.
A wave of French blue-chip companies have abandoned their profitability targets for the year, while employers' associations have warned that hundreds of smaller firms and shops risk bankruptcy.
The estimated 35 per cent reduction in normal economic activity "appears in line with preliminary data available on employee situations, with one-third still working as normal, one-third working from home, and one-third unemployed," Insee said.
