The day he saw the king have sex with his mother is a childhood memory that will never leave Angel Cristo.

Barbara Rey made a paella on the terrace of her family home for Juan Carlos before the couple frolicked in the pool.

The fun continued to the bedroom where Ms Rey and the long-serving Spanish monarch rekindled a romance that had begun nearly 20 years earlier.

But while Juan Carlos was indulging in an extra-marital affair behind the back of his wife, Queen Sofia, his mistress was engaged in her own act of skulduggery – one that would allegedly net her millions but also traumatise her teenage son.

Before the king arrived on that sunny afternoon in 1994, Mr Cristo, aged 13 at the time, was covertly stationed on their neighbour’s side of the garden fence.

He was tasked with capturing the liaison on his Canon EOS 700 camera.

The Telegraph has today published a selection of those photographs.

Mr Cristo also said he made edited copies of an explicit video showing Ms Rey, an actress and TV host, and the then King of Spain having sex in her bed.

The footage was recorded on a secret video camera installed by his mother.