[Your letterhead, if desired; if not, your return address]
[Date of letter—month, day, and year]
[Recipient's first and last names]
[Street or P.O. box address]
[City, State ZIP code]
Dear [recipient's name]:
I am writing to let you know that I have not yet completed the work project that was due to you today. I sincerely and profusely apologize for this delay and for any problems it might cause for you or for the rest of the team members. I had set aside a certain number of hours to complete the work, and unfortunately, I had not anticipated that the work would take as long as it has taken. I anticipate that I will complete the work by two days from now and will send the completed project to you sooner if I am able to do so more quickly.
Thank you for your patience, and again, I apologize for any inconvenience caused by this delay.
[Sender's first and last names]