Marine Le Pen's recent decision to spend less time talking to the news media in 2016 is an error that "excites ambition in other potential candidates who would ... endanger your chances of reaching the second round" in the 2017 poll, Le Pen tells his daughter in the letter.
And he goes on to hint that he might form a new party, if his warnings are ignored.
"If our efforts fail, aware as we are of the terrible dangers that loom over our party, we will not give up and we will, with regret, act outside the Front National," the letter says.
Marine Le Pen announced the lowering of her media profile after a strategic seminar of party leaders at the beginning of the month.
Le Pen senior declares that the meeting "gave birth to a mouse" with no notable changes to the party's programme, although "a few rats" had tried to "gnaw away" at his legacy, "one wishing to scrap the name Front National, another to stop the 1 May march".
Another idea that was rejected was a proposal to drop the commitment to pulling out of the euro.
The far-right veteran's letter is a not-so-veiled threat to officially found his own party that would stand against the FN in elections.
After his expulsion in August he set up an organisation, the Blue-White-Red Rally, but declared that membership should not be incompatible with FN membership and in December's regional elections he supported his niece, Marion Maréchal-Le Pen, who had replaced him as leader of the party's slate in the Provence-Alpes-Côte d'Azur region.
Like her uncle, Maréchal-Le Pen is unhappy about the party's bid to win disillusioned left-wing voters by taking up social issues - a line identified with FN vice-president Florian Philippot - and says that most of the party's votes in her region came from defectors from the mainstream right.
"The FN should play its role as the backbone of a great national impetus," Jean-Marie Le Pen says in his letter. "One should not lose part of one's political capital in the hope of conquering others. You have to be yourself."