* Problems in National Front since election setbacks
* Marine Le Pen at odds with deputy, Florian Philippot
* National Front struggling to be main opposition voice
* Philippot a strong advocate of party's anti-euro line (Adds FN official, political analysts)
PARIS, Sept 20 (Reuters) - Four months after a heavy electoral defeat, France's National Front is on the verge of a split between its leader and her chief deputy that could open the way to a policy change on the euro as the far-right party seeks to rebrand itself.
Marine Le Pen's relationship with deputy Florian Philippot, for years her closest aide, has become so strained since she lost the presidential election to centrist Emmanuel Macron that far-right experts say it has reached the point of no-return.
A rupture would mark an important change for the FN and Le Pen because she has for years defended Philippot's anti-euro, protectionist line against critics in the party, and built her 2012 and 2017 presidential campaigns on his advice.
Philippot has been under fire in the party over its setbacks in this year's presidential and parliamentary elections, with many blaming the anti-euro line he has backed so strongly.
Le Pen, whose leadership is also under scrutiny, has increasingly distanced herself from Philippot, focusing on the party's anti-immigration, identitarian roots in speeches rather than the party's stance on the euro.
Philippot has stood firm, maintaining that opposing the euro is a vote winner.
Le Pen has now issued him an ultimatum, saying he must decide between his party role and a think-tank group he has set up focused on an anti-euro policy.
"I've asked Florian to choose. I'm going to meet him. If he doesn't accept (to choose), I'll choose for him," Le Pen told the CNews digital channel on Wednesday, adding that he needed to make up his mind quickly.
"He's created a conflict of interest. If we can't settle the differences, I'll take the necessary decisions."
Jerome Fourquet, a specialist on the far-right at pollster Ifop, sees the split as definitive.
"This seems to have escalated to a point of no-return," he said.
If Philippot leaves the party or is forced out, it could at first cause further turmoil for the far-right as it struggles to portray itself as the main opposition to Macron, despite having reached the second round of the presidential election in May.
In the presidential run-off, Le Pen secured 34 percent of the vote against 66 percent for Macron -- its strongest election performance but still a disappointment. In June's parliamentary election, the national Front won only eight seats.
The role of chief opponent has been assumed by Jean-Luc Melenchon, leader of the far-left France Insoumise (France Unbowed), who together with unions is leading calls for mass protests against reforms planned by Macron.
The departure of Philippot, a graduate of France's top ENA administrative school, could be followed by others in the party with a similar profile walking out, depriving the party of figures who were key to policy-making.
Le Pen and her party might, however, see long-term gains as his exit could enable her to reassert her authority and make it easier to shift policy on the euro. This might in turn help the FN forge alliances with other right-wing politicians.
"If Philippot was to leave there would be a negative buzz, there would be turmoil in the FN, but after a while that would likely be digested and overcome," Fourquet said.
He underlined that the party had survived previous crises, such as the departure in the late 1990s of Bruno Megret, once a political star who took allies with him. Analysts say Philippot has fewer loyal allies and a smaller political capital than Megret did at the time and would therefore be unlikely to create a rival party that would pose a serious threat to the FN.
The deadline for the FN to decide how it wants to rebrand itself is March, when it will hold a party congress to decide on a new name and what its strategies and policies should be for the next presidential election due in 2022.
Jean Messiha, who coordinated Le Pen's presidential manifesto, said the party was in a difficult phase where it was adapting to France's changed political landscape.
"We need to get our act together and, most importantly, we need Marine Le Pen to reaffirm, to reassert her authority," he told Reuters.
Le Pen's partner, senior FN official Louis Aliot, made a similar point, telling supporters at the weekend not to give in to "defeatism".
"In politics, you're never dead," he said. (Additional reporting by Simon Carraud and Dominique Vidalon,; Editing by Luke Baker and Timothy Heritage)