Locks have been changed at the Tower of London, home to the Crown Jewels, after a thief was spotted by guards stealing a set of keys from a sentry post.
But the famous Beefeaters reportedly did not give chase. Instead they radioed for help, as rules prevent them leaving their posts.
Tower chiefs insisted that the priceless Crown Jewels were never at risk of being stolen but admitted security procedures "were not carried out to the expected standard".
Police have launched an investigation after the intruder stole the set of keys from a sentry box at the fortress site.
The keys gave access to the Tower's drawbridges as well as conference rooms and a restaurant, according to Historic Royal Palaces, which operates the tourist attraction.
A Historic Royal Palaces spokeswoman said: "We can confirm that an incident took place in the Tower environs in the early hours of November 6 and that this is now a police matter, so we are unable to discuss details.
"We can however confirm that during this incident, keys for a restaurant and conference rooms were taken, together with a key to an internal lock to the Tower drawbridges that is not accessible from the outside.
"It would not have been possible to gain access to the Tower with any of these keys. All the affected locks were immediately changed.
"We have carried out an internal investigation and have concluded that our well-established security systems and procedures are robust.
"However on this occasion, these procedures were not carried out to the expected standard. A staff disciplinary procedure is under way to address this issue."
A Scotland Yard spokesman said: "An allegation of theft has been made to police, which is being investigated by Tower Hamlets CID."
The Tower of London is famously guarded by the Yeoman Warders, nicknamed Beefeaters, along with a private security firm.
The Yeomen first formed the Royal Bodyguard in 1509 but their origins go back to the reign of Edward IV in 1461.
Serving Beefeaters must have completed at least 22 years military service, and are synonymous with the Tower - as are the six resident ravens.
Legend has it that the kingdom and Tower will fall if the ravens - each of which has a single clipped wing - ever leave the high-security site.
The Crown Jewels have had an allure for countless visitors over the years and the British monarchy is the only one in Europe that still uses the symbolic gems in coronation ceremonies.