Tuesday's 600-page agreement between the UK and Brussels was a "great step", Philippe said on Thursday during a visit to Dunkirk centred on the effects of the UK leaving the EU on France's Channel ports.
But, as four ministers resigned from May's cabinet and MPs laid into her in the House of Commons, Philippe warned that "Nothing at this point permits us to know if the agreement will be adopted in the end."
"It will have escaped nobody's notice that the current British political situation could give rise to a certain number of questions and concerns about the effective possibility of the ratification of this accord," he said.
France does not want a no-deal exit but the possibility is "still on the table", Philippe added.
France prepares no-deal backup
The French parliament is preparing a bill in case that eventuality materialises and has identified 200 measures, in particular relating to customs and sanitary control, that need to be taken before the Brexit deadline of 29 March, according to the prime minister.
France is working with the European Commission on the question and bilaterally with Ireland, Belgium, the Netherlands and Germany.
Earlier Le Maire hailed this week's plan as "good news for the French economy, good news for all French companies".
"It's in everybody's interest that Brexit passes off peacefully," he declared.
French ministers are particularly hopeful that finance houses will leave the City of London for Paris so as to be able to work within the European Union, although the city faces competition from Germany's finance capital, Frankfurt, in that domain.
Le Maire, too, called for "prudence" until the moment the deal is signed.
"If it stays in the customs union, we must ensure that Britain respects all the European rules", especially the "fiscal rules" and "environmental norms", he said, adding that "It must weaken our single market."
EU summit in November
Government spokesman Benjamin Griveaux sounded a cautious note after the French cabinet met on Wednesday.
"This is an encouraging sign compared to what we have seen over the last few weeks and months," he declared. "But we are obviously still very prudent. We will take the time needed to examine this document in detail."
An extraordinary EU summit to agree the deal is planned for 25 November but it must be ratified by the British and European parliaments.
An opinion polls this week found that 13 percent of French nationals living in the UK intend to leave after Brexit, while 62 percent definitely wanted to stay.