An IELTS 8 Bands Letter

You successfully passed a job interview. You are expected to start on November 15, but you will not be available on that date.
Task: Write a letter to your new boss, explaining your situation, expressing your concern and suggesting solution.

Dear Sir/Madam,

I am Aadil Mukhtar and I have recently passed a job interview for the Software Engineer position at the Software Infrastructure group. I was expected to start on November 15, as agreed, and am writing because unfortunately I will not be able to present myself on that date.

During the interview I estimated fifteen days as the amount of time needed for me to finish my activities at my current job and start at your company, but due to fact that I got sick for the last whole week and was not able to work, I could not finish my tasks and will need another week to do so. If necessary, I can give you a copy of a letter from my doctor informing that I had to stay at home and under medical care during that week.

I really apologize for the inconvenience and expect it not to affect our relationship. Should I present any other form of proof or talk to someone else at the company, please let me know.

Faithfully yours,
Aadil