I found the following in the book "The Most Common
Mistakes in English Usage" published by McGraw-Hill
and written by Thomas Elliott Berry, Professor of English,
West Chester State College, West Chester, Pennsylvania:
An authoritative ruling has now been established concerning
the use of "can not" and "cannot." The correct form is now
almost universally considered to be the single word "cannot."
Wrong: I can not attend the meeting.
Right: I cannot attend the meeting.
The negative of the auxiliary can, cannot occurs less frequently than the contraction canít but much more frequently than can not, which is sometimes used for emphasis and to reflect the fact that this locution may be stressed on the not as well as on the can. Cannot is Formal, canít relatively Informal in writing, and canít is the more frequent in Conversational contexts, but it is often appropriate as well even in some Oratorical uses. All three forms are Standard. Cannot, like canít, is a very frequent replacement for may not.