The online auctioneer Catawiki explains where the brick has come from:
The LEGO brick is made of 14-carat solid gold and was gifted by LEGO between 1979 and 1981 to a specially selected number of business partners and employees, who had worked with LEGO for 25 years. It is estimated that there are only ten of these bricks in existence.
Five years ago one such brick sold for around £11,000, a record that the auction website expects to be broken when bidding closes tonight. The gold brick is going for £4,768 at the time of writing.