Townshend was arrested in January on suspicion of making and possessing indecent images of children. The arrest was part of Operation Ore, an FBI-led crackdown on Internet child pornography.

After a four-month investigation, London’s Metropolitan Police said Wednesday that the 57-year-old rocker “was not in possession of any downloaded child abuse images,” but had accessed a site containing such images in 1999.

The musician acknowledged using his credit card to enter a Web site advertising child pornography but said he was doing research for his autobiography.

At the time of his arrest, Townshend said he was not a pedophile and had campaigned against child pornography.

The title character in Townshend’s rock opera “Tommy” – a deaf, mute and blind pinball wizard – is sexually abused by an uncle, and Townshend said he believed he had been sexually abused as a young boy, while in the care of his mentally ill grandmother.

On Wednesday, Townshend acknowledged he had been wrong to access the Web site, but said police had accepted he had no “nefarious purpose” in doing so.

“As I made clear at the outset, I accessed the site because of my concerns at the shocking material readily available on the Internet to children as well as adults, and as part of my research toward the campaign I had been putting together since 1995 to counter damage done by all kinds of pornography on the Internet, but especially any involving child abuse,” he said in a statement.

Police said it was not a defense “to access these images for research or out of curiosity.”

As part of the cautioning procedure, Townshend’s fingerprints, photograph and a DNA sample will be taken by police, and he will be placed on a national register of sex offenders for five years.

Townshend was one of The Who’s four founding members, along with bassist John Entwistle, singer Roger Daltrey and drummer Keith Moon. Moon died in 1978 and Entwistle died last year.

The group, founded in London in the early 1960s, was part of the first British rock invasion, alongside the Rolling Stones and The Beatles. Their hits included “I Can See for Miles,” “Pinball Wizard” and “Won’t Get Fooled Again.”