Brinkley tells of husband's affair with teen – and £2,000-a-month porn habit

The former supermodel Christie Brinkley told a court yesterday how her marriage fell apart when she discovered her fourth husband was having an affair with a teenage employee and spending almost £2,000 a month on online pornography.

Giving evidence in the second day of a salacious divorce battle which has gripped America, Miss Brinkley said she found out about her husband Peter Cook's affair with 18-year-old Diana Bianchi when the girl's stepfather confronted her at a high school graduation ceremony where she was guest speaker. Ms Bianchi was working as a personal assistant to Mr Cook, 49, at his architectural practice in Long Island, New York.

Ms Brinkley, 54, said Ms Bianchi's stepfather, a police officer, tapped her on the shoulder and said: "That husband of yours won't knock it off. He's having an affair with my teenage daughter." She told the court: "My life as I knew it had vanished."

The former Sports Illustrated model said she looked at her husband sitting in the front row of the hall at Southampton High School and thought: "My God, it's true. He did do that." She recalled: "That was the day that my world was completely shattered."

Ms Brinkley, whose second husband was the singer Billy Joel, intends to call 44 witnesses during the case against Mr Cook at Central Islip Court, where she is seeking custody of their son Jack, 12, and daughter Sailor, 10. Under New York state rules, she must prove Mr Cook was at fault in the relationship.

The estranged couple's lawyers are disputing ownership of a portfolio of property on Long Island, where the couple were members of the super-rich Hamptons set. They are also arguing about who should keep their fleet of boats.

But it is Ms Brinkley's insistence that the case is heard in open court which has polarised opinion in the US. Mr Cook's lawyers claim she is a woman scorned, whose demands to go public are merely hurting her children; she claims her children deserve to know the truth behind their parents' break-up.

During the first day of the trial, it emerged that Mr Cook met Ms Bianchi in early 2005, when she worked as an assistant in a toy shop. He said their affair began that March, about the time he hired her as his assistant. He paid her £10,000 to type magazine articles on to the company's website and also left her cash payments in various spots: £250 under a rock outside his office; more behind a painting. Before their relationship ended in late 2005, they had sex in his office and at homes in the Hamptons owned by Ms Brinkley. In May 2007, Ms Bianchi was given £150,000 by Mr Cook as an inducement not to reveal the affair.

It also emerged that Mr Cook spent £1,800 a month on internet pornography which, his lawyer said, he and his wife used to get themselves in the mood for sex. Ms Brinkley, however, claimed her husband visited sex websites to "try to connect with people".

Having persuaded Mr Cook to hand over his computer password, arguing that he had "nothing to fear if he had nothing to hide", she said she was shocked to find not only emails from Ms Bianchi but a sexual contact site visited by Mr Cook. "It was not a voyeur site," she told the court. "It was a meeting site. It was more than I could bear."

Meanwhile, the acrimony between the couple spilled out of the court and into US television studios yesterday, with lawyers from both sides trading verbal blows on behalf of their clients. Mr Cook's attorney, Norman Sheresky, disclosed details of a psychiatrist's report that has yet to be put before the judge, which concluded that Ms Brinkley was "consumed by rage".

"[It] has found that Christie is angered and she is going to have to overcome her anger," Mr Sheresky said, adding that the psychiatrist believed Mr Cook's pornography use was inadmissible as evidence. "Peter is a wonderful father. He should have a relationship with the children and Christie Brinkley should not interfere with it," he added.

"What has hurt these children is the fact that Christie Brinkley is mad after two years. She is absolutely furious and she wants to get even. Unfortunately, she's getting even by hurting the children. If you look at her past marriages, you will find out that she believes that when her marriages fail, she gets to keep the children. That's not the law in this state."

Ms Brinkley's lawyer, Bob Cohen, hit back, saying of Mr Cook: "He acted very, very badly over a long period of time with a high-school senior 30 years his junior. I can't imagine any less appropriate custodial parent than Peter Cook." The case continues.