Marine Le Pen's niece, who became the youngest member of France's national assembly aged just 22 in 2012, told Vaucluse Matin daily in a letter to appear Wednesday that she was leaving politics "for some time."
She explained she "wishes to work ... (in) the business world" while also spending more time with her family, being the divorced mother of a young child.
Known in the party simply as Marion, she said she did not want "to hang on at all costs" to the status afforded by her parliamentary seat, dealing a blow to the National Front ahead of parliamentary elections next month.
But she insisted that she was not definitively turning her back on political struggle. "I shall never be able to remain indifferent to the suffering of compatriots," she said in her letter to the paper.
As well as leaving parliament, Le Pen is stepping down as opposition leader on the council for the southern region of Provence-Alpes-Côte d'Azur.
Her grandfather is Jean-Marie Le Pen, who stood for the presidency five times. He called her move a 'desertion'.
He reached the second round run-off vote in 2002, where his losing margin to the centre-right Jacques Chirac was even greater than the 66 percent to 33 percent margin which Macron earned over Marine Le Pen on Sunday.
Marion had previously hinted at dissatisfaction with the internal party politicking and her relationship with both Marine Le Pen and the party's deputy chairman Florian Philippot, who has taken the FN in a more "social" direction to the chagrin of its more conservative wing.
'Alone and isolated'
Marine had suggested her niece was too "inexperienced" for a ministerial post in the event of victory over Macron while Philippot described her in December as "alone and isolated" in the party.
Marion urged the party to "reflect" on her aunt's presidential defeat and future strategy. Despite garnering a record for the FN of some 10 million votes, Marine Le Pen was unable to secure enough support from the traditional right or undecided voters.
In another development on the French right, Eric Woerth, tasked by the centre right Les Republicains (LR) with strategy for next month's legislatives, said he hoped to see "co-existence" with Macron and the latter's centrist movement founded only last year.
Woerth said co-existence would be the byword for relations with Macron supporters rather than "opposition" or "hard co-habitation" -- that being an allusion to scenarios in the late 1980s and the mid-1990s which saw a leftist president twice forced to coexist with a conservative dominated parliament.
The reverse then happened between 1997 and 2002 when Chirac, having served as prime minister under socialist Francois Mitterrand, had to contend with a leftist government.
For Woerth, "Macron brought oxygen and freshness" into a presidential contest which saw the candidates of the traditional parties bite the dust in the first round of voting, before Sunday's runoff.