The online auction site eBay 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."
He added that the company invests more than £6 million every year "in developing the best technology possible to prevent anything from slipping through the net".
"As the world's largest marketplace we don't handle items directly, so we use sophisticated technology to help us identify high risk or illegal items," he said.
"Safety is our number one priority and we recognise we need to do more to protect our members.
"Therefore, we are reviewing our position with regards to the sale of all knives on eBay, including those currently permitted by law."
