Error 1075 when dropping primary index

[Wim Roffel]

When I try to drop the primary index of a table like

ALTER TABLE `mytable` DROP PRIMARY KEY

I get the error:

MySQL error 1075: Incorrect table definition; there can be only one auto column and it must be defined as a key

I tried to search the internet. It seems to be related to the fact that the field is an AUTO_INCREMENT field. But it evades me why this is relevant. And I would like to find a way to drop the key anyway.