The teenage star of a British film in competition in Cannes will miss her red carpet premiere tonight because she has just had a baby.

Katie Jarvis, 17, gave birth to a baby girl, Lily Mae, on Saturday.

The father is her long-term boyfriend, Brian, with whom she was arguing when spotted by a casting director for Andrea Arnold's gritty drama, Fish Tank.

Unveiling the film this morning, Ms Arnold said she did not know what Katie would have made of the fuss at Cannes.

But her acting debut in the film has already secured the young star an agent and acting roles she has so far been unable to take on because of her pregnancy.

Ms Arnold said Katie had been “really brave in so many ways” during the strange and demanding experience of long days' filming.

“But she really took it in her stride. On some level it was water off a duck's back.”

Katie was having an argument with her boyfriend on the opposite platform of the station at Tilbury, where she lives in Essex, when a casting assistant was in the town looking for someone to play 15-year-old tearaway Mia.

Katie did not originally believe the approach, but took a number and eventually called.

“It was an interesting experience for her because she has never done anything like this before,” the director said. “She had not long been out of school and hadn't done any work.”

Filming required long days as Katie appeared in nearly every scene on the shoot around Barking and Tilbury.

“She got very tired. When you've done filming before you know that at weekends to pace yourself you go to bed. But she was going out to parties or going out and buying loads of shoes with the money she was earning,” Ms Arnold said.

The director last saw her shortly before the birth when she had just been featured in the Evening Standard in an article on Cannes.

But Ms Arnold said she did not think the teenager really understood the significance of festivals like Cannes. Looking around a packed press conference, she added: “I have no idea what she would have made of this.”

In a statement, Katie said: “It was hard, but it was fun and rewarding. Now I want to make the most of it. It shows you don't have to go to drama school to get into it, but I think I was one of a kind. I don't think anyone else will get picked off a train station.”

The film follows the impact on Mia when her mother (Kierston Wareing) gets a new boyfriend, played by Michael Fassbinder, the star of last year's Cannes award-winner Hunger.

Andrea Arnold, 48, said she was “quite anxious” about being in Cannes despite winning the jury prize here three years ago for her debut feature, Red Road.

She  first grabbed headlines when she described winning an Oscar for her short film Wasp as “the dog's bollocks”.

Fish Tank, backed by BBC Films and the Lottery, is in contention for the prestigious Palme d'Or against directors including veteran Brit Ken Loach, Quentin Tarantino, Ang Lee and Lars von Trier.