By Maggie Fick

NAIROBI, April 27 (Reuters) - Kenyan authorities have banned a film that tells the love story of two women and is set to debut at Cannes Film Festival next month, saying it promotes lesbianism.

The film "Rafiki", a word that means friend in KiSwahili, was this week invited to premiere at the prestigious festival in France -- the first Kenyan film to receive to be invited.

On Friday, the Kenya Film Classification Board said it had banned the film.

"Anyone found in its possession will be in breach of law," the board said in a tweet, referring to a colonial-era Kenyan law under which gay sex is punishable by 14 years in jail.

Board spokeswoman Nelly Muluka tweeted: "Our culture and laws recognize family as the basic unit of society. "The (board) cannot, therefore, allow lesbian content to be accessed by children in Kenya."

Film director Wanuri Kahiu said: "I'm really disappointed because Kenyans already have access to watch films that have LGBT content, on Netflix, and in international films shown in Kenya and permitted by the classification board itself."

"So to then just ban a Kenyan film because it deals with something already happening in society just seems like a contradiction," she told Reuters.

Homosexuality is taboo across Africa and people who are gay face discrimination or persecution. In recent years, however, campaigners for lesbian, bisexual, gay and transgender rights have become increasingly vocal.

The ban coincides with a landmark case brought by gay rights campaigners to repeal Kenya's law on gay sex on the grounds that it deprives sexual minorities of basic rights.

The film is adapted from an award-winning short story "Jambula Tree" by Ugandan writer Monica Arac de Nyeko.

The film commission used a hashtag, #KFCBbansLesbianFilm, that immediately sparked a barrage of supportive tweets from Kenyans who decried homosexuality. (Reporting By Maggie Fick Editing by Matthew Mpoke Bigg)

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