Lily's little brother Alfie to become the latest famous Allen

Alfie Owen-Allen, the younger brother of the singer Lily Allen, is to star in the psychological stage drama Equus after his performance in a marginal film role so impressed the play's scriptwriter, Peter Shaffer, that he ordered its producer to recruit him.

Shaffer, who wrote the play in 1973, was struck by Owen-Allen's acting abilities in the film adaptation of Ian McEwan's novel Atonement, in which he plays the minor character Danny Hardman.

He will replace the Harry Potter star Daniel Radcliffe, who played the lead in the West End revival to critical acclaim earlier this year.

David Pugh, the producer, said Shaffer phoned him after watching Atonement to say he had found the perfect actor for Equus.

When Owen-Allen auditioned for the part, his portrayal of the main character, a disturbed teenager who blinds horses, was so dramatic that it moved the play's director, Thea Sharrock, to tears.

As the 21-year-old brother of Lily and the son of the actor and comedian Keith Allen, he is the youngest member of the family to prove himself as an exceptional talent.

Owen-Allen will star opposite Simon Callow in the play, which will begin a nationwide tour at the Chichester Festival on 31 January, and may return to the West End after that.

While Radcliffe will continue to play the lead in the play on Broadway next autumn, producers had encountered difficulties in finding a fitting replacement for the UK tour.

After auditioning nearly 200 young men across the country, they were left exasperated, until Shaffer's discovery, according to Mr Pugh. "We interviewed a lot of TV actors. I spent four months auctioning 184 boys and we didn't know where to go. Then Peter Shaffer saw this young actor who he thought was really special. When we auditioned him, we all realised we had found a little gem," Mr Pugh said.

Lily Allen dedicates a song to her younger brother, called "Alfie" , on her debut album, Alright, Still, whose lyrics describe him as somewhat lazy.

Having heard the song, Mr Pugh was initially apprehensive about Allen's professionalism. "Peter [Shaffer] and I agreed to meet him for breakfast at 8am and Peter said, 'If he turns up at 8, he has got the part, but if he turns up later, he hasn't.' When we arrived, Alfie was sitting there, already having ordered his bacon and eggs," said Mr Pugh.

Owen-Allen, who has has a host of minor characters in several films including Elizabeth, produced by his mother, Alison Owen, is now undergoing training for his first lead role, which requires on-stage nudity. Mr Pugh did not dismiss the idea that Owen-Allen may also appear on the New York stage, if he proved a success in Britain.

Alright, still, the family

* Lily Allen, sister

Pop star whose single "Smile" reached No 1 in the UK last year. In December, debut album Alright, Still was nominated for best British album at the Brit Awards.

* Keith Allen, father

The Welsh-born actor appeared in The Comic Strip Presents... films on Channel 4 in the 1980s. Currently starring in the television series Robin of Sherwood.

* Alison Owen, mother

Film producer at Ruby Films and former wife of Keith Allen, she is the force behind such cinema successes as Elizabeth and Shaun of the Dead.

* Kevin Allen, uncle

Film and TV actor. Directed Agent Cody Banks 2: Destination London.