The rock star Pete Townshend was yesterday cautioned by police and placed on the sex offenders register for five years following his admission that he accessed child pornography on the internet.

Townshend, 57, received the caution at Kingston police station in south-west London for "accessing a website containing child abuse images".

He was arrested in January as part of Operation Ore, the largest investigation into child pornography in the UK. He admitted using his credit card to access images but claimed they were for "research" for a book.

Scotland Yard said in a statement: "At 12.00 today the musician Pete Townshend was formally cautioned for accessing a website containing child abuse images in 1999.

"After four months of investigation by officers from Scotland Yard's child protection group, it was established that Mr Townshend was not in possession of any downloaded child abuse images.

"He has fully cooperated with the investigation.

"As a routine part of the cautioning process fingerprints, a photograph and a DNA sample will be taken; additionally in these cases, the person concerned will be entered on the sex offenders register for a period of five years."

In a statement released by an aide outside his mansion in Richmond, south-west London, Townshend insisted that police had "unconditionally accepted" that he was looking at the site as research for his "campaign" against child pornography.

"From the very beginning I acknowledged that I did access this site and that I had given the police full access to all of my computers," said the former member of the Who.

"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 towards 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.

"The police have unconditionally accepted that these were my motives in looking at this site and that there was no other nefarious purpose, and as a result they have decided not to charge me."

The Scotland Yard statement stressed that access and payment for child abuse images was an offence.

"Inciting others to distribute these images leads to young children being seriously sexually assaulted to meet the growing demands of the internet customer. It is not a defence to access these images for research or out of curiosity."

Child abuse campaigners condemned the leniency of Townshend's punishment, and said he should get professional help.

Jennifer Bernard, from the children's charity NSPCC, said: "Every child seen on an internet pornography site is a real child who is likely to have been abused time and time again.

"Only a quarter of children report sex abuse and many feel that they have no one to turn to.

"People who pay to access these sites are injecting cash into a criminal and manipulative industry that sexually exploits and seriously damages children."

The children's charity NCH said Townshend's crime was "not a small matter".

John Carr, the charity's internet safety adviser, said: "It is not an acceptable defence and it only helps to keep the child porn industry going."