The former Miss World is due to appear at the Cannes festival as a debate about her weight rages in her home country

Aishwarya Rai is no stranger to Cannes. The Bollywood actor and former Miss World has attended the film festival 10 times, but her appearance on the red carpet next week is set to be her most talked about. The cause of a controversy raging in India lies not with her latest film – Rai is there to promote a cosmetic brand rather than a new movie – but instead, bewilderingly, with recent photographs which suggest that, six months after giving birth to a baby girl, Rai has yet to regain her pre-pregnancy figure.

Many in India are asking whether the woman routinely referred to as the most beautiful in the world, and who occupies a place in Indian popular culture akin to Kate Middleton or Victoria Beckham, has an obligation to her fans to lose weight.

Born on 1 November 1973 in Mangalore, Karnataka, in the south of India, Rai was the second child of a merchant navy officer. Her modelling career began while she was still at school; at 18 she won the Ford supermodel contest, which led to her being cast in a television advertisement for Pepsi. In the 1993 commercial – which also featured future Bollywood heart throb Aamir Khan – Rai appeared for only three seconds but her pouting cameo brought her national fame. The following year she won Miss World. "That contest is seen as a bit of a joke in the west but Rai winning really mattered to India," said Rachel Dwyer, professor of Indian cinema at the University of London.

Rai then made the predictable move into acting, but with a surprising debut: Iruvar was a Tamil political thriller rather than a Bollywood blockbuster. She went on to star in more mainstream films such as Jeans, Hum Dil De Chuke Sanam and, most notably, Devdas, which brought her global attention. Unlike heroines from the 70s and 80s who were often healthily curvy, Rai's popularity rested on a body that was supermodel lean.

"Most of the films she has appeared in – with a few exceptions – have been critically trashed," said film critic Mayank Shekhar. "Her prime talent – if not her only one – is that the west perceives hers to be the most beautiful face to have come out of India."

"She is an icon because of her extreme beauty," said novelist Kishwar Desai, "and also because she has broken through to the international market and that means a lot to Indians."

Julia Roberts, in a quote endlessly recycled but never substantiated, is reported to have described Rai as the most beautiful woman in the world and it was the interest from the United States – where Rai was invited to appear on both The Oprah Winfrey Show and The Late Show with David Letterman – and from Britain – where she starred in Bride and Prejudice – that made her Bollywood's biggest star. In 2003 she was invited to join the Cannes jury alongside Steven Soderbergh and Meg Ryan. The decision, critics suggested, was motivated more by marketing than qualification – a suspicion not entirely laid to rest by Rai's comment on arriving at Cannes: "I am here to learn."

Shekhar said: "One has to make a distinction between her career as a public performer and as a film actress. Her career as a public performer is more successful than her film work."

This public role included not only acting but also brand endorsements, charitable work and a high-profile marriage. Rai is said to earn more than £10m annually for endorsing such brands as Longines, Lux and L'Oréal – for whom she is appearing in Cannes. Alongside the commercial endorsements, she also lends her fame to social causes. She has committed to donating her green eyes to blindness charity Eye-Bank after her death. Last month it was announced that a school named after her is being built for girls in a remote village in Uttar Pradesh.

She also stays in the headlines because of her eventful love life. Before her marriage she was in a relationship with actor Salman Khan, which ended acrimoniously with Rai releasing a press statement saying: "I have endured his alcoholism and misbehaviour, and I have been at the receiving end of his abuses – verbal, physical and emotional, infidelity and indignity." Khan denied the allegations.

In 2007 she married another actor, Abhishek Bachchan, the son of Amitabh Bachchan, India's most loved film star. Human rights groups criticised Rai for promoting a Hindu tradition associated with the caste system by 'marrying' a tree ahead of the wedding – apparently to overcome astrological differences with her fiance. On the wedding day 300 policemen were stationed around their Mumbai home and one young woman, who claimed she was Bachchan Jr's former girlfriend, slit her wrists on the street outside and required 40 stitches. The wedding sealed Rai's admission into the Bachchan dynasty. "She had it all," said Desai. "Picture-perfect looks, winning Miss World and then marrying the most eligible bachelor in India by marrying into one of this country's most respected families: it is a fairytale life."

The criticism of Rai's post-pregnancy figure has been fierce . "Aishwarya is like a goddess," said Showbusiness columnist Shobhaa Dé. "She is held up as the ideal of beauty and so there is an expectation on her to look perfect at all times."

The image that sparked the storm in India showed Rai in the back of a car on her way to a party being thrown for the UN secretary general, Ban Ki-moon, where Ban is believed to have asked her to be the global face of the UN's girl child campaign. The Indian media, though, was less interested in the job offer than the fact that Rai looked like a 38-year-old woman who had recently given birth.

One website posted a video, complete with elephant sound effects, entitled "Aishwarya Rai's shocking weight gain", which has been seen more than 500,000 times. "She is a Bollywood actress and it is her duty to look good and fit," suggested one commenter. Another added: "She needs to learn from people like Victoria Beckham who are back to size zero weeks after their delivery."

The criticism has started a debate about Bollywood's attitude towards women and motherhood. "There is a glorification of motherhood in India and Indian cinema," said cinema professor Shohini Ghosh. "But people are confused because they don't know whether to glorify Aishwarya in her new motherhood or lament that she is not looking like a runway model."

India has long grappled with the question of what beauty means, but in an age of global media Indians are familiar with western celebrities and how quickly some of them appear to regain their pre-baby bodies. "The role models being held up are Angelina Jolie and Victoria Beckham," said Dé. "But our body frames are different – we have wider hips and curves – so this whole business of looking desperately skinny two weeks after giving birth is a western import."

The career trajectory of female Bollywood actors has been downwards after they married and had children. From 50s star Nargis to Rai's mother-in-law Jaya Bhaduri, who largely retired after marrying Amitabh Bachchan, the assumption was that Indian audiences were not willing to accept as a screen heroine a woman known to be married and a mother. But there are signs of progress: this month two other women – Madhuri Dixit Nene and Karisma Kapoor – are making film comebacks after having children.

Rai's appearance at Cannes could be an important cultural moment. "It could be a turning point in making us stop and review the absurd expectations we have of our female celebrities," said Dé.

"The timing is right for us to start learning to accept women with a few more curves and appreciate that gorgeous women do age," said Desai. "But the question is would Aishwarya want to age gracefully in front of us and will we let her?"

Potted profile

Born: 1 November 1973 in Mangalore, Karnataka

Career to date: model, Bollywood actor, humanitarian

High point: winning Miss World 1994, winning FilmFare best actress award twice, besting David Letterman on his US TV chat show

Low point: after ending her relationship with fellow actor Salman Khan she told the Bombay Times: "There were times when Salman got physical with me, luckily without leaving marks. And I would go to work as if nothing had happened."

What she says: "I always knew I would be successful. So there was no element of surprise."

What they say, allegedly: "The most beautiful woman in the world" (Julia Roberts)