The letter, sent to Buckingham Palace by the Duke and Duchess of Sussex's office, is thought to have been passed on to the independent legal team.
"Broken" royal aides described feeling "sick", "terrified" and left "shaking" with fear. Others were said to have been reduced to tears by the duchess.
After the palace launched the probe, one alleged victim said: "We will finally be able to tell the truth. It's not going to be easy, but this is very welcome and long overdue. We don't have to be silent any more."
The palace originally insisted that the review would be in-house and led by HR director Elisabeth Hunka. The Sussexes have called the claims a "calculated smear campaign based on misleading and harmful misinformation".
Buckingham Palace said: "Our commitment to look into the circumstances around allegations from former staff of the Duke and Duchess of Sussex is being taken forward but we will not be providing a public commentary on it."