The teenager was taking part in a guided tour of the Assembly building in Cardiff when he accessed the computers to send a joke e-mail to a friend.

Christine Chapman was shocked when she received the e-mail

But the prank backfired when the message was sent to Assembly Member Christine Chapman instead.

Ms Chapman, who had recently written an anti-racism column for a newspaper, was shocked when the message flashed up on her laptop computer as she worked from home.

She immediately contacted security officials at the Assembly, who managed to track down the culprit.

He was given a warning and allowed to continue with the tour.

'A minor review'

A spokesman for the Assembly confirmed that a security review had been ordered to prevent visitors accessing desktop computers in the chamber.

He said: "The Presiding Officer has written to all members asking them not to bring visitors to the Members' area of the chamber because of concerns about the security of the technology there.

"These computers do not hold any files but it is possible to send e-mails from them.

"This incident highlighted that we did need to look again at our security arrangements, although it is only a minor review."

'Badly spelt'

Ms Chapman, who is Labour AM for the Cynon Valley, said: "When I first saw the message it really unnerved me. It was not a nice thing for them to do."

Her researcher Simon Evans added: "It was a one-line message which shook her up a bit, but once the shock had passed it became clear it wasn't a truly sinister message.

"The message was badly spelt, which pointed to it being children.