It was the story everyone wanted to tell. When 11 middle-aged Yorkshirewomen from the Rylstone & District Women's Institute took their clothes off for charity five years ago, scriptwriters across Britain and the United States were itching to take up their pens and tell this eccentric English tale.

The worldwide success of the calendar they produced, and the huge popularity of the film The Full Monty, meant the women's story was marked out from the first as the basic material of an international cinematic hit.

But Calendar Girls, the British film which has taken Cannes by storm, has had a secret and troubled path to the big screen. The work of the original writer, who first had the idea for the film and won the trust of the WI, was substantially rewritten just before filming began with the comic approach favoured by its Hollywood backers, Buena Vista International.

Juliette Towhidi first took her idea for a film to the small British production company Harbour Pictures four years ago after reading about the calendar in the British press. She and producer Suzanne Mackie befriended the WI members, who all live in the Dales village of Cracoe, six miles from Skipton, and persuaded them they would tackle the story sensitively.

'They chose us because they trusted we would handle John's [the husband of one of the women] death properly,' said Towhidi this weekend.

So strong were the bonds that were built up, the women turned down rival offers direct from Hollywood and from the comedian Victoria Wood, who made them a higher offer than any other British independent.

The 11 women were split. Some were concerned to keep Hollywood out, while others wanted to get the best financial deal. All the women's royalties from any film were to be donated to the Leukaemia Research Fund, because their risqué calendar had been inspired by John's death after a long battle with the disease.

After a ballot, Towhidi and Harbour Pictures won by five votes to four and writing began. Towhidi's script focused on the death at 54 of John Baker, the National Parks Officer played in the film by John Alderton.

But the switch to a new scriptwriter just before shooting moved the emphasis away from Baker's death and towards the experiences of the WI women who later travelled out to promote the nude calendar in America. Following sales of 88,000 copies in Britain, it became a huge seller in the States, where it outperformed both Britney Spears's calendar and the infamous Sports Illustrated, although some of the women's nipples were airbrushed out in the US edition. Half the 'calendar girls' were flown to Los Angeles and appeared on a series of top-rated talk shows.

Tim Firth, the screenwriter brought in to re-write Towhidi's version, was asked to inject more humour.

'I told them I can't even tell jokes, let alone write them,' he said. 'But I started to think how these women would talk. I think Juliette was perhaps too close to the story.'

Firth, whose mother and grandmother are WI members, is the writer behind the current West End hit musical Our House .

'Harbour Pictures and Buena Vista were having problems with the ending and came to me. I did two weeks' work and didn't even read the first script until I had finished,' he said.

Tricia Stewart, who came up with the calendar plan and is played by Helen Mirren, said she had trusted Towhidi and Harbour Pictures and had not been involved in the last-minute decision to turn it into more of a comedy.

'We weren't involved in any of these decisions, so I am just grateful it has come out so well. A lot of it has been fictionalised now - I haven't got a teenage son, for example - but the heart is still there.'

Mirren, who was in Cannes to promote the film with Walters and the real-life characters they play, Stewart and Baker, said she had read both versions of the script and was pleased by the changes. 'The first read-through of Tim's script was really funny and I think the humour reflects the women themselves. I did not respond to the first script as much as I did the second, but I hope there are not too many jokes now. The comedy still comes from the truth about the characters.'

Producer Mackie said the film had not been 'Hollywoodised' but admitted that making it had been a painful process at times.

A one-off payment to the Leukaemia Research Fund, agreed by Harbour before they made the £7m film is to be followed by a 1 per cent share of all profits. The calendar has raised more than £500,000 for leukaemia charities and the Rylstone & District WI plan a follow-up for 2004.