No one likes to be blind-sided so, if possible, give notification that a project or report will be late before it is actually late. Take responsibility and be straightforward in communicating the problem. Blaming others will not make the situation any better.
From: Jim Karter
Subject: <subject of memo>
I regret to inform you that <name of project or report> will not be completed by <date of deadline>. I believe it will be finished by <new expected completion date>.
The <reason/reasons> for delay <is/are> <reason(s).>
I believe we can make the <new expected completion date> date. I'll keep you posted on any further developments.