Credit card companies are turning detective in order to help police track down the large number of people entering child pornography websites. When rock star Pete Townshend gave his credit card details to view indecent images of children, for what he claims was research, he left a trail found by the police months later.

The card number - which had been scrambled but was decoded by the US Postal Information Service along with thousands of others - inevitably led back to him.

Now card companies are using the same method to track down those providing the pornography.

Visa has an internet monitoring programme costing hundreds of thousands of pounds a year.

It's a constant battle and we continue trying to stop the use of Visa cards for this type of abuse Jon Prideaux Visa

It seeks out millions of websites to trace those which charge users from their credit cards to access illegal material.

Visa spokesman Jon Prideaux told BBC Radio 4's Today programme: "Visa does not agree with Visa cards being used for child pornography and we do a lot to prevent it.

"We have a programme which is scanning a programme of about one million websites a day in order to find those sites which may be involved in child pornography."

Mr Prideaux said about 30,000 suspicious websites were initially identified, then that number was reduced to a few hundred after further examination.

He said: "Most of them have been closed down and the rest are in the process of being closed down.

"But it's a constant battle and we continue trying to stop the use of Visa cards for this type of abuse."

Fooled

Visa submits the website names to the police, but said it does not have the names of individual users.

"The programme we've got is very much looking at the people who are selling this, from a merchant perspective.

"We're not really looking from the cardholders' side of the equation, it's much more difficult to pick that up.

"We can obviously go away and look, and say: 'Well, here's a site that's accepting it', but we can't necessarily go in and raid it."

The police need to get a move on and the government needs to finance it as a matter of urgency Deborah Shipley MP

He said although Visa did not know who the clients were, the police could track them down via the websites.

Mr Prideaux said banks were sometimes fooled into collaborating with a porn website, when the content of the site can initially appear to be quite innocent.

But it is only one point of control, he conceded, because many pornographic websites did not use credit cards.

Labour MP Deborah Shipley, who campaigns on child protection, told the same programme: "I welcome what Visa is doing, but I'd like to know are all credit card companies doing that.

Townshend says he was researching a book

"Those that aren't doing it I would like to see blacklisted by all major companies in this country, and refuse them their credit."

But the problem for police would appear to be prosecution, rather than detection.

They have only arrested 1,300 people from a list of 6,000 suspected offenders.

Deputy assistant commissioner Carole Howlett, spokeswoman on internet child pornography for the Association of Chief Police Officers, last year asked for more police resources to help.

Ms Shipley said £500,000 had been given, but the police needed about £2m to tackle the huge scale of the problem.

"The police need to get a move on and the government needs to finance it as a matter of urgency," she said.

Although users could throw away their computers to try and escape detection, the credit card evidence may be enough to nail them, she added.