The sale of large numbers of Blue Peter badges on eBay is being investigated by the BBC.

The badges, which come in five types and are awarded to contributors to the show, can get the holder free entry to stately homes and museums. They have been presented to luminaries including Brad Pitt and Madonna.

The BBC says its investigations unit has been monitoring the situation for months, with editor Richard Marson explaining: "We know how hard children work to earn a badge, and we are doing our best to ensure that this long-standing Blue Peter institution is not undermined."

As of Monday morning there were 18 badge auctions on eBay.