Under French law a candidate needs 500 signatures from elected officials in at least 30 different departments across the country or in France's overseas territories.
Le Pen says she is still short of the target, telling a TV show on Saturday she was up to 415 signatures.
In the letter, published by newspaper Nice-Matin on Wednesday, Bardot said she was scandalised that mayors have had "cold feet" so far.
She asked them to lend their support so that Marine Le Pen could be part of the elections, which are due to be held in two rounds on April 22nd and May 6th.
Bardot, who now devotes her time to animal causes, said Le Pen "defends animals and has the courage to give our country, France, its proper place in the world."
"I therefore ask them [mayors] to have a bit of courage for once in their lives and to do their duty," she wrote.
There was disappointment for Marine Le Pen on Tuesday when the country's top constitutional authority ruled that the signatures received by presidential candidates should continue to be published.
Le Pen and some other candidates from smaller parties had argued before the court last week that those offering to give their signatures should retain their anonymity.
"If proof was needed that I am the candidate of the people against the system, I'd like to thank the Constitutional Court for giving me that today," Le Pen said after the decision.