Emmanuel Macron has said it's up to the UK alone to decide how they leave the European Union.
However, he's said the Irish backstop is an "indispensable guarantee" to protect stability in Ireland and the integrity of the EU's single market.
The French president was speaking as Boris Johnson arrived in Paris for talks about Brexit amid the ongoing deadlock over the Irish backstop.
While the British prime minister has repeatedly claimed that the backstop must be dropped in any deal, European leaders have said that can't happen without realistic alternatives.
Angela Merkel - who Mr Johnson met yesterday evening - has suggested any solution should be found in the "next 30 days", describing the backstop as an "expression of a problem we have not yet solved".
Mr Macron today said the measure is not just a technical constraint, but instead a "genuine, indispensable guarantee to preserve stability in Ireland the integrity of the single market".
The French president argued: "I've always been clear - a choice was made, and we cannot just ignore it. We'll have to implement the decision taken by the British people - I want to be efficient, and I very much want all of us to find a solution.
"That being said, I have to be very clear: we will not find a withdrawal agreement within 30 days which will be very different to the existing one.
He added: "If what Michel Barnier has negotiated can be amended while complying with the integrity of the single market... then we can find a solution.
"If not, it's probably a political decision for the prime minister - it will not be our decision."
Speaking alongside Mr Macron outside the Élysée Palace, Mr Johnson once again said he wants a deal and is optimistic one can be reached.
He suggested he was "powerfully encouraged" by discussions in Germany yesterday with Angela Merkel and her officials.
However, he claimed the UK must leave the EU on October 31st with or without a deal.
Mr Johnson insisted: "Under no circumstances will the UK government be instituting or imposing checks or controls of any kind at [the Irish] border."
'Miracle' needed
Foreign Minister and deputy Prime Minister of Luxembourg, Jean Asselborn, is pictured during the ZDF talk show 'Maybrit Illner' on the topic 'Chaos, Clowns and Euro Crisis - is Italy dragging us into the Abuyss' at the ZDF studio at Berliner Zollernhof Unter den Linden in Berlin, Germany, 07 March 2013. Photo: Karlheinz SchindlerMeanwhile, Luxembourg's foreign minister today expressed scepticism that a solution to the current impasse will be found.
Speaking to German broadcaster SWR (translated by Reuters), Jean Asselborn argued: "Miracles should never be ruled out, but I’m sceptical we can simply pluck something out of the air that guarantees Ireland has no hard border and at the same time the EU has control over what enters its market.
"I’m not convinced that we can for sure say today a no-deal Brexit is coming, but we must clearly outline the dangers of a no-deal Brexit and Boris Johnson should do so too."
Mr Johnson yesterday claimed there are “abundant solutions” to the issue and suggested the UK government had not “actively proposed” them over recent years.
He said he was “more than happy” to hear the German Chancellor set a “blistering timetable of 30 days” to overcome the deadlock.
