The online auction site eBay has admitted it needed to do more to protect its members after illegal knives were sold on “the world’s online marketplace”.

Researchers on the BBC’s Watchdog programme bought five knives, including a stealth knife and a dagger disguised as a pen, on the site, all of which are illegal to sell or carry in the UK.

The investigation comes just days after Home Secretary Jacqui Smith launched an initiative with more than 20 retailers to ensure that knives are sold responsibly.

An eBay spokesman said: “Trust is the engine of eBay and as the UK’s number one e-commerce site.

“We know we have a responsibility to keep the site safe for our buyers and sellers.

“Although the UK site did not have any illegal knives for sale by UK sellers, it was possible for UK buyers to see or purchase knives from international sellers.

“This should not have been possible and we have taken steps to close that loophole.”

The spokesman added that the company invests over £6m every year “in developing the best technology possible to prevent anything from slipping through the net”.

Alf Hitchcock, deputy assistant commissioner of the Metropolitan Police and the leading national police expert on knife crime, told Watchdog that eBay had a moral responsibility to make sure that people were not being killed or injured by the weapons.

“I find it quite shocking that a retailer, and a responsible retailer at that, would be selling offensive weapons,” he said.

The knives, which all cost less than £10, were sold by US sellers through eBay’s site in the UK.

The Watchdog investigation will be shown on BBC One at 7.30pm today.