Lynne Ramsay's debut feature, Ratcatcher, looks - on the grimy surface of things - like Dirty Realism taken to an absurd extreme. If you want grit, here it is: a panorama of greasy bin liners, toxic skin rashes and sudden death in stagnant canal water.

But Ratcatcher is also one of the most poetic films we've seen in a long while - certainly the most intense vision of the everyday to emerge from Britain in an age. Since its debut at Cannes, its 29-year-old Glasgow-born director has been hailed as Scotland's answer to Ken Loach, Terence Davies and Robert Bresson, as well as Scotland's most distinctive screen voice since the 1970s, when the late Bill Douglas made his autobiographical trilogy.

This is the sort of acclaim that can turn a film-maker's life upside down; with Ratcatcher released in Britain this week, Ramsay is still only just recovering from the experience of making the film and following it round the festivals. "Your personality changes when you make a feature," admits Ramsay, a diminutive, intense chain smoker, dressed in severe black chic, but given to bouts of gleeful giggles. "It has repercussions, and you've got to stop yourself losing the plot."

In 1990s Britain, young film-makers have tended to aim straight for the commercial jugular. But Ramsay has shown her resolve by making something profoundly unfashionable - a personal statement that's downbeat and dream-like, impressionist rather than story-driven, and defiantly rooted in a place and culture. Set in a Glasgow tenement during the refuse collectors' strikes of the 1970s, Ratcatcher is a picture of childhood lived at subsistence level among decay, violence and burgeoning sexual curiosity. But despite the surface severity, the film is intensely uplifting for its humour and the sheer grace of Ramsay's visual imagination.

The story and its setting are close to the director's own early childhood in the Glasgow area of Maryhill, but she shot Ratcatcher in another part of Glasgow, in Govan. "The street we were going to shoot in was going to be closed and knocked down, and people were meant to be re-housed while we were shooting, but none of them were. Some people were very suspicious, but we had a good location manager who built up a relationship and a lot of people were extras. It felt more like a community project than a film. But it was a very rough area, really dodgy. It happened to be a housing area for families that had been difficult for various reasons. People were saying, `we're going to steal your equipment,' but we never had any trouble."

Ratcatcher's early 1970s setting is heavily over-colonised these days: it's a foreign, more lurid country that filmmakers tend to visit with amused fondness. But Ramsay creates a sense of time and place with no condescending kitsch - this is the 1970s of Play for Today, rather than Top of the Pops.

"I think that's the reason we did catch the mood," she says. "People in that environment in the 1970s didn't wear flares; maybe they were wearing hand-me-downs. That stuff makes things look really period, and I didn't want that - I wanted it to be hard to place. What was strange for me was to see the way the family worked - the dad going to work, the breadwinner. But that still exists quite a bit in the place we shot in - and the poverty. The wallpaper's changed, but not that much has."

One of Ramsay's strengths is working with child actors - real children and not a rosy-cheeked cherub in sight. Ratcatcher's lead is William Eadie, a solemn 12-year-old with an ancient, bony pallor. When Ramsay casts children, she says, "It's partly about the face, and about attitude - just the disposition, the un-self consciousness. I always go for unusual kids."

Ramsay's feel for childhood and its attendant traumas steered her through Ratcatcher's more delicate scenes, particularly those involving the sexually precocious teenager Margaret Anne, played with pinched intensity by Leanne Mullen. One scene in particular led to Scottish tabloid attacks on Ratcatcher as an underage sex film.

"That scene's very innocent, actually," says Ramsay. "Maybe there's an element that's latently sexual; really it's two kids having a laugh in the bath. When you're a kid, you do things that are pretty risque - the kind of things that no one will admit to their mothers and fathers but we all did. I was very lucky to get that scene - but it was the toughest I've ever shot."

A graduate in photography at Edinburgh's Napier university, Ramsay belatedly took to cinema through discovering the poetic work of filmmakers like Bergman, Bresson, Tarkovsky and particularly the enigmatic shorts of Maya Deren. Enlisting at the National Film School, she learned her trade as a cinematographer but felt out of step working on other students' commercially-angled scripts. "I only started writing because I thought, `I don't relate to these scripts, how can I shoot them if I don't believe in them?' So I chose more carefully than the directors. It was a training for me - like public school."

Ramsay's graduation film, Small Deaths, was also her first Cannes prize-winner - a triptych of vignettes pinpointing early episodes in the death of the soul. She followed it with Gasman, a poignant story about a child discovering her father has another family on the side, and Kill the Day, her most experimental short, a fragmented portrait of a heroin-addicted petty thief. Its lead is Ramsay's saturnine older brother James and it was a difficult film for both of them, the director says. "Because he's got such a past, he brings that to the screen - there's a lot happening without him doing much, it just comes out. He's a very cinematic character because of that."

The American independent company Good Machine has now commissioned her - along with directors like Hal Hartley and Gaspar Noe - to contribute to a series on sex, entitled Uncensored. "The brief mentioned things like Ai No Corrida and Last Tango. The challenge is to make pornography that is actually involving. I want to do something completely without moral judgment." She is also busy adapting Morvern Callar, Alan Warner's novel of emotional blankness in the Balearics.

The novel's amoral glamour should satisfy those who have been urging Ramsay to take on more commercial projects: "Morvern Callar could be completely existentialist, but it's got a young girl of 21, some rave scenes and some sex scenes - lots of selling points. I think I'm becoming less naive about how everything works."

Ramsay may not be naive, but she can be intransigent. She says she fought over everything from Ratcatcher's ambivalent ending to the poster campaign. "I can't sleep at night if I feel something's a compromise, so I put more stress on myself - I put myself through the whole deal."

In the end, she says, the film is "90 per cent of what I wanted, and I think I got a lot more than most people get. But I need 100 per cent, I need the whole thing. I can't bend into a malleable shape, because I won't make anything good that way." If Ratcatcher really is only 90 per cent of what she imagined, I can't wait to see what happens when Ramsay is given that long leash she's intent on finding.