“尽快回信”常见的英文表达有：
Reply as soon as possible：这是最直接、常用的表达，“reply”意为“回复”，“as soon as possible”就是“尽快”的意思。例如：Please reply as soon as possible.（请尽快回信。）
Write back at your earliest convenience： “write back”表示“回信”，“at your earliest convenience”意思是“在你方便的最早时间”，整体表达更委婉、礼貌。例如：I'd appreciate it if you could write back at your earliest convenience.（如果你能在方便的最早时间回信，我将不胜感激。）
Get back to me at the earliest opportunity： “get back to sb.”有“给某人回话、回信”的意思，“at the earliest opportunity”即“一有机会就；尽早”。例如：Could you get back to me at the earliest opportunity?（你能尽早给我回信吗？）