Authorities say the Crown Jewels and other unique artifacts are safe but that locks had to be changed after an intruder broke in and stole some keys.
Officials said Monday the keys were stolen from a sentry box at the site. They would have provided access to the Tower's drawbridges and some other areas.
Authorities say the keys were stolen last Tuesday and a police investigation into the theft is under way.
A Historic Royal Palaces statement said security procedures had not been properly followed.
The Tower is guarded by Yeoman Warders, commonly known as Beefeaters.