France's first lady, Carla Bruni-Sarkozy, is to launch herself on the international humanitarian stage tomorrow with an announcement that she is to become a figurehead in the global fight against Aids.

Bruni, who married President Nicolas Sarkozy in February, was expected to be appointed an “ambassador” for the Global Fund to Fight Aids, Tuberculosis and Malaria, a Geneva-based organisation backed by the United Nations.

She had for long been trying to decide what cause to embrace as first lady, and the choice of Aids appears to have been influenced by a family tragedy that occurred in 2006, when Virginio, her brother, with whom she had a close relationship, died after a long battle with the illness at the age of 46.

Her latest album features a song called Hi, Sailor, which has been described as a farewell to Virginio, an artist and passionate yachtsman, who had frequently crossed the Atlantic single-handed.

His death devastated the family: Carla has spoken of the “emptiness” it caused, and in 2007 Marisa, her mother, set up the Virginio Bruni Tedeschi Foundation in his memory. Backed by Unesco, it is dedicated to fighting Aids.

French media were debating whether Carla’s new ambassadorial role would allow her to perform in concerts to raise money for the cause.

Proceeds from her latest album have been donated to charity but, besides singing the occasional song on television, she has yet to give a public performance of it.

With her sparkling smile, elegant designer wardrobe and flashes of wit, the 40-year-old Bruni has enchanted the public, all the more so since adopting French citizenship and abandoning her Italian passport. She is credited with exerting a calming influence on the volatile Sarkozy.

Foreign audiences seem even more appreciative of the raven-haired singer with the long legs and whispery voice who posed in a red dress for one publication on the roof of the Elysée Palace. “If I were Italy, I’d try to get you back,” David Letterman, the American chat show host, recently told her.

After a timid beginning as première dame, she appears to be growing in confidence, risking a diplomatic incident recently by chiding Silvio Berlusconi, the Italian prime minister, for his joke about America’s president elect, Barack Obama, being “tanned”. She said: “I often feel very happy that I am French.”

Since Obama’s election she has also backed a petition calling for more racial equality in France, where politics, entertainment and business have for long been dominated by a white, Parisian elite. On the Letterman show she called it “an honour” to be France’s first lady: “I just want to be good enough. But I also like it. I try my best.”

Commenting on her relationship with Sarkozy, 53, she described it as “love at first sight” and suggested that there had been little option other than to get married.

“The people don’t really like it when the president is dating,” she said, referring to a dip in his popularity rating during their whirlwind romance.

It was on her first overseas trip as first lady, when she accompanied Sarkozy to South Africa in February, that Carla originally broached the idea of getting involved in humanitarian work. After that she became bogged down in the recording and marketing of her album and her humanitarian projects had to be put on hold.

No other first lady, with the possible exception of Michelle Obama, can offer such promise as a humanitarian figurehead, and even before beginning her role she has notched up some notable successes.

One example of the “Carla effect” was the extraordinary boost she gave to a women’s cooperative that she visited in a township near Johannesburg in February. The women gave Carla one of the handbags they make out of jute and cotton. She liked it so much that she gave them permission to market it as the “Carla bag” in Paris. It has since become an indispensable fashion accessory, selling for €100 (£80) at Colette, a chic Parisian boutique.

In her new role, she is expected to work closely with Michel Kazatchkine, the French director of the Global Fund, a famous immunologist who has been engaged in the battle against Aids since 1983. Sarkozy is believed to have been extremely supportive of her decision and has encouraged her to speak out in public. “She’s a highly intelligent woman,” said an aide. “He trusts her.”

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