I am afraid that…
I am sorry that I missed the opportunity of contacting you earlier/ calling you…
I am very sorry to have caused you so much trouble…
I must apologize that…
I regret to inform you that…
Please, accept my apologies for…
Please, forgive me for troubling you taking so much time / taking so long to answer your letter the delay in…
The reason for the present delay is…
Unfortunately …
We are sorry for the inconvenience that we may have caused you.
To my regret I must inform you that…