The Merlin Conspiracy is a children's fantasy novel[3] by Diana Wynne Jones, published by HarperCollins in April 2003, simultaneously in Britain and America.

The King's Progress is a mobile Court that continuously roams the Islands of Blest (our England) to contain and control the natural magic in the world.

The Merlin Conspiracy is set in the same multiverse as the 1997 Jones novel Deep Secret, although the only characters in common are Nick and his father Ted Mallory.

The New York Times gave cautious praise to this novel, saying it "blends some New Age spirituality with the grand old Stonehenge/White Horse/Arthurian/Britain-as-Magicland material into a strong concoction", but felt that the characters could be more vivid and the mythology more inventive.

The header of its review includes the note "(Ages 10 and up)", evidently provided by the publisher or the US imprint Greenwillow Books.