A man who used computer chatrooms obsessively and has been called "Germany's first internet killer" has gone on trial. After flirting with more than 300 women online, and meeting many of them, he admits that he met and killed two of the women – one of whom was stabbed 26 times with a bread knife.

Using the pseudonym "Rosenboy", the accused, a builder named only as Christian G, 27, spent years scouring the internet for women with whom he hoped to have casual sex. His defence lawyers said he created an online world that became a substitute for reality.

Yesterday, Christian G told judges in the Ruhr city of Essen that he had killed two of his chatroom acquaintances last summer.

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His case has shocked Germany and earned G the nickname "Germany's first internet killer" in the tabloid press. In contrast, Armin Meiwes, the German cannibal who was convicted of killing and eating a man in 2004, advertised for a willing victim on the internet. Christian G, who appeared in court yesterday with a goatee and close-cut hair, told judges he had killed his first victim, Jessica K, 26, "by mistake" after the two quarrelled during their initial meeting last June. Prosecutors alleged he stabbed her with a knife.

Chrstian G claimed to have killed his second chatroom date, a 39-year-old mother of three called Regina B, 12 days later because she had "provoked" him by asking him for money. Prosecutors said her corpse had been found with 26 stab wounds which seem to have been inflicted with a bread knife.

In a prepared statement read out by his lawyer, Christian G told the court that, for several years, he had spent most of his free time visiting chatrooms. "Over the years, I built up at least 300 contacts through these chatrooms. I met at least a hundred, if not a hundred and fifty, of these women personally," he said. "I spent the whole night in these forums and thought of them as my family," he added.

His lawyer said that, after his arrest last year, Christian G had repeatedly asked for information about what his online contacts had to say about him on chatroom sites once he had been put on trial for murder.

Christian G said he had met Jessica K in her home town of Stade, near Hamburg, in early June last year and had intended to have "sex in the open air" with her. He said that, during a quarrel that occurred, he had grasped her "tightly by the throat", and that afterwards she had simply "fallen to the ground."

However, prosecutors have accused Christian G of killing Jessica K with a knife. They said that forensic scientists had found traces of her blood on his shoes. The accused has argued that the blood stemmed from a nose bleed Jessica K had suffered during their brief meeting. Her body was found two weeks after her death and police said it had been impossible to determine an exact cause of death.

Christian G said he had killed his second victim, Regina B, after he had had sex with her and she had demanded he pay her €500. He said that, when he refused, the two had taken Regina B's dachshund for a walk and she then called him a rapist and threatened to bring rape charges against him.

In testimony, Christian G claimed to have suffered a mental blackout when he subsequently attacked her. "I felt as if I was under remote control," he said in his statement. "I can only say that if Regina had not said these things and accused me of being a rapist, she would be alive today," he said.

The trial continues.