Tesco and Asda have come in for criticism for stocking books and DVDs with adult or offensive titles on their websites, where they could be available to children.

Asda apologised to the Mail on Sunday after the newspaper complained that titles containing the f-word were readily available on its website.

Tesco, meanwhile, admitted there had been lapses in its online filtering system, designed to prevent offensive titles appearing on its main site.

“Is this Tesco’s first step into the adult retail market?” said Conservative MP Nadine Dorries. “What kind of supermarket with a shred of moral responsibility allows such products to be sold openly on the internet, available to children, possibly without a parent’s knowledge?”

Liberal Democrat MP  Don Foster warned: “If the industry can’t collectively sort itself out then we must seriously look into external regulation. If they can’t regulate themselves, we may have to introduce a statutory code.”

“We block material [that] may cause offence so that titles like these cannot be searched for or accidentally found,” said a spokesman for Tesco. “Unfortunately our filter process was not working properly but has now been fixed. We’re grateful this was brought to our attention as we do take this responsibility seriously.”

An Asda spokesperson commented: “We sell more than 750,000 items on our website, of which a vast majority are books and music, and these few titles slipped through our filter system.”