A late reply is one of the most common Japanese business-email fixes learners need. The goal is to own the delay, stay proportional, and move to the point — not to stack the heaviest keigo you know. The examples below are live API rewrites (Business / client).
A light delay (same day / one day)
Input
Sorry for the late reply. About the meeting — are we still on for Thursday at 3pm?
Natural Japanese
お返事が遅くなり申し訳ございません。会議の件ですが、木曜日の午後3時に予定通り実施いただけますでしょうか。
Short apology, then the meeting ask. No stacked 何卒／申し上げます closings on a light check-in.
Rewritten with Tadashii Kotoba — Business style, recipient: client (live API).
A heavier delay (several days / blocked work)
Input
I am very sorry I took so long to reply. Your work was blocked because of me. Here is the file you asked for.
Natural Japanese
ご返信が遅くなり、大変申し訳ございません。私の不手際により、お客様の作業を滞らせてしまい誠に恐縮でございます。ご依頼いただいておりましたファイルを送付いたしますので、ご確認いただけますでしょうか。
When the delay hurt the other side, the rewrite adds weight (大変／恐縮) and still delivers the missing file in the same message.
Rewritten with Tadashii Kotoba — Business style, recipient: client (live API).
What to avoid
Long self-blame paragraphs, repeated させていただく stacks, or blaming busy-ness without an apology all land poorly. One sincere line beats three ornate ones — let the rewrite match the weight of the situation.