Mr Johnson said: “As Hans Haacke’s take on the equestrian statue trots into Trafalgar Square, it brings another reason for Londoners and tourists to visit this cultural landmark. Gift Horse is a startlingly original comment on the relationship between art and commerce and I hope it will stimulate as much debate as the other works that have appeared on the Fourth Plinth.”

^{The Fourth Plinth is unveiled at Trafalgar Square (Rex Features)}

The plinth, in the north west corner of the square, was built in 1841 to hold an equestrian statue of King William IV but the money ran out and it was left empty.