The panic button system is similar to one that the networking site Bebo has already announced and involves a prominently displayed means for children to report offensive, obscene and other inappropriate material.

The standards should also give parents greater control over children’s use of the internet with controls that enable them to prevent access to pages with offensive content.

Sites will also have to offer a “safe search” service that stops children from inadvertently accessing unsuitable sites.

According to research by industry regulator Ofcom, about 35pc of children are able to access the internet in their bedrooms without parental supervision.

The Sunday Times reported that Ed Balls, the schools secretary, and Alan Johnson, the home secretary, would unveil the voluntary code on Tuesday and that Gordon Brown might also attend the launch.

Websites which are moderated by staff are the focus of the new guidelines, particularly internet chatrooms, search engines and instant messaging services.

Further details are being working on by the UK Council for Child Internet Safety (UKCCIS) and these are due be published next summer.